Discussion on tire efficiency
 

Hello, folks,

Two discussions about tire efficiency have been going for a few days. This is an effort to consolidate into one clearly-labeled thread, and maybe give some useful information. The following "Lesson" on tire efficiency was created (by me) a few years back and became a presentation at a national TDI Club meeting.

I know I will offend a few folks with different views. And occasionally I may make errors so feel free to correct me. Here is the presentation, without substantial change--

Lessons on Fuel Economy
#1 Tires /Ernie Rogers

The purpose of this discussion is to help a car owner get the best possible fuel economy from his car, any car. At times I may talk about parts that are difficult to change, but mostly I will suggest ideas that are easy to do. Let’s talk about tires and some general principles about fuel economy. Later, I promise to write about some other topics, including air drag.

GENERAL PRINCIPLES

It takes fuel to make power. So your car consumes fuel in proportion to the power delivered to the wheels. The power needed by the wheels is the force to keep moving multiplied by how fast you are going. We can make an equation out of this—

We’ll measure the fuel flow in gallons per hour.
Define a proportionality constant for how much fuel is needed, C1
Call the force to move the car F(pounds) and the speed of the car v(miles per hour).

Putting the information together, we get:

Gallons per hour = C1 F(pounds) v(miles per hour).

If you divide the Gallons per hour by the v(miles per hour), you get something that is a measure of fuel economy:

Gallons per mile = C1 F(pounds)

Gallons per mile is the inverse of miles per gallon. This little formula teaches a powerful lesson about how parts of the problem are related to fuel economy. It says you should add up all of the forces opposing the motion of the car, and multiply that total force times C1, a number representing various other aspects of car efficiency such as the energy content of the fuel, the efficiency of the engine, and other losses from things like the gears and bearings, etc.

Looking at fuel economy in gallons per mile is helpful because we can see how the drag forces (and other forces) add together to cause consumption of fuel. (In Europe, the normal fuel economy units are liters per 100 km, a very nice choice.) Let’s use gallons per mile for awhile, and when we want we will switch to the inverse, miles per gallon.

With fuel prices getting way out of line, we would like to lower both C1 and F. Today’s discussion about tires involves one part of the total F. The total force the car must fight has four parts, representing air drag, energy to turn the wheels and tires, energy for acceleration, and energy to overcome gravity when going uphill. *

F = air drag + wheel rolling resistance + mass times acceleration + uphill grade.

To summarize, C1 is some constant related to the fuel and the efficiencies of various car parts (and of course they aren’t always exactly constant). C1 multiplied by the forces opposing the car’s motion gives a good measure of fuel economy. The forces can be looked at one at a time and then added together.


TIRES AND ROLLING RESISTANCE

The force due to energy losses by the tires is the “rolling resistance.” Even choosing a set of tires has a scientific basis and can be demonstrated mathematically, for those who remember some algebra. There is a simple formula for rolling resistance, Frr:

Frr = Crr Mg

This discussion will be about how to make the rolling resistance smaller. The first factor, Crr, is the “rolling resistance coefficient,” a commonly-used “constant” to indicate the efficiency of tires. Mg (mass times earth gravity) is the weight of the car. In truth, the rolling resistance coefficient can vary a little bit with speed so it’s not really constant, and other things besides the tires can be involved—but let’s ignore such details.

The first obvious way to lower rolling resistance is to throw away non-essential parts of your car, to lower the weight. Let’s skip that option for now. That leaves finding ways to change the tire characteristics to lower their essential efficiency property, the rolling resistance coefficient. One might ask, “how much does my car’s miles per gallon depend on Crr?”

Answer: For ordinary car tires, the rolling resistance coefficient varies from about—

Crr = 0.012 (cheap tires) to Crr = 0.0060 (very good tires).

So, rolling resistance can change by as much as a factor of two, depending on your tires. For a modern, efficient car, the effect on fuel economy is—

Cutting Crr in half causes—
at 60 mph: 14% better mileage
at 40 mph: 20% better mileage.

Well, this is very, very good. The tires on my own car were the best I could find (I think), and I measured their rolling resistance coefficient as Crr = 0.0065. You should be able to find several tire makes with Crr equal to 0.009 or better. It’s an unfortunate state of affairs that, at least so far, tire manufacturers seem intent on keeping their Crr values a secret. That could mean taking the time to evaluate tires for ourselves. We need to change that. Some tire properties that affect efficiency are things you can change without buying new tires. Let’s talk about them. Here is a list of tire properties that are expected to affect Crr:


Tire Properties

Profile (P, given as a percentage)
Tire pressure (p), which is connected to--
Contact patch area
Diameter (D) : related to P,
tire width (w) and
wheel rim diameter (d)
Rubber stiffness (depends on temperature)
Rubber internal friction (depends on temperature)

Here is some of the information you can find on the walls of my tires:

Michelin Energy MXV4 S8 205/60 R16 91V Max Press = 44 psi

The first part says something about the design and materials of construction. I know that the Energy MXV4 S8 tires have a rubber composition with very low internal friction, and that means very efficient. The tire size, 205/60 R16, can be used to calculate the approximate diameter of the tire. It tells me that the tire width is w = 205 mm. The “/60” is the tire profile, indicating the ratio of height to width, is P = 60%. The tire has radial cord placement, and the mating wheel rim diameter is d = 16 inches. Here’s how to calculate the tire diameter:

D = 2 ( w/25.4 ) P + d

So my tire diameter is: D = 2(205/25.4)(0.60) + 16 = 25.7 inches.

“91V” indicates the load and speed rating of the tire. Unless you are an unusually crazy driver, tires with the lower rating are just fine. In fact, tires with lower speed and load rating will usually be a little more efficient. (They have less rubber and cord to absorb energy.)

The maximum pressure (stated on the side of your tire) is the maximum safe gauge pressure when the tire is “cold,” before you have started to drive the car. “Cold” actually means the expected air temperature while you are driving—best viewed as the maximum air temperature until the next time you check your tires. Tire pressure changes with temperature, but not a lot. If the tire pressure is set to 35 psi at 70 degrees F, then the pressure will be 34.1 psi at 60 degrees and 35.9 psi at 80 degrees. So, a good rule of thumb would be to decrease tire pressure setting by 1 psi for every 10 degrees F that you think the air temperature will be higher while driving than it is while setting the pressure.

Of course, once you have driven your car for a few miles and the tires have begun to heat up (because they have internal friction, the thing we hate), you really don’t know the temperature of the air inside the tires. That’s why tire pressure should be measured “cold,” before you have driven more than about two miles.

Following are my impressions about how tire characteristics affect Crr, and then also their effect on mileage.

Profile

Engineers call this the “aspect ratio” of the tire. A large number like /65 tells you that the side wall is very tall, at 65% of the tire width. The experience of many drivers indicates that a tire’s efficiency declines with its profile number. I believe this is consistent with theory, since a long flexible side wall allows more give in the tire with less local flexing. Alternate flex and rebound of the walls and tread as a tire rolls is the main source of energy loss in a tire. I think the lowest profile a tire can have and be very efficient is P = 55%.

Tire Pressure and Contact Patch Area

A car is held up entirely by the air pressure in its tires, except for just a few pounds used to deflect the tire rubber a very small amount. Each tire forms a flat spot on the ground, called the “contact patch.” This portion of the tire transmits just enough force to exactly balance the load it supports. From this, you can calculate the area of the contact patch:

mg = (Contact Area) (Gauge Pressure)

mg is the weight supported by the tire, which equals the product of its contact area and its pressure. The amount of flexing of the tire is about proportional to the area of the contact patch. That means the tire rolling resistance increases as the tire patch area increases. Experience shows, however, that rolling resistance doesn’t increase in proportion to the contact area. The explanation is that a tire remains cooler at higher pressure, and the cooler rubber loses somewhat more energy than warmer rubber, all else being equal.

Many of us that are intent on obtaining the best fuel economy we can will operate our cars with somewhat higher tire pressure than is recommended by the car manufacturer. I set my tires at 40 psi cold, or about 10% below the maximum pressure stated on the tires by the tire maker. I have had no problem with uneven tire wear at the higher pressure (for good-quality tires), and the tires are found to last much longer than you would expect. The downside is that the high pressure makes the ride too bumpy for some people’s tastes.

Diameter

Tire diameter affects a lot of different aspects for a car. In most cases, increasing tire diameter makes things better.

Increasing D provides:

Lower rolling resistance
Higher engine efficiency
Longer tire life
Smoother ride and improved control.

Theory suggests that rolling resistance is proportional to one over the tire diameter. The reason large D is better is that there is less deflection of the rubber to get the needed contact area. In addition, most cars obtain best engine efficiency at lower engine RPM than you get with standard transmission gearing and normal-sized tires. That’s because car companies make cars the way people like them—with lots of acceleration—“perky,” as I might say. Engine RPM is lowered for a given speed when you increase tire diameter.

The standard factory tire size for my car was 205/55 R16, with a tire diameter of 24.9 inches. Changing the profile from 55% to 60%, to tire size 205/60R16, increased my tire diameter by 0.8 inches, a 3.2% increase in diameter. This small change has given about a 5% increase in fuel economy for my car. So far, I have driven 55,000 miles on these larger tires, and the tread wear has only been 3/32 of an inch. I still have at least another 3/32 of rubber before I need to think about replacing my tires. (The original tread depth was 9/32: the tires now have 5/32 to 6/32.) Michelin Energy tires are not known for giving long wear, unless you have larger-than-normal tires, as I do.

Pluses and minuses of increasing tire diameter: better mileage, tire life and handling, but less perky at the stop sign. Your speedometer and odometer may be less accurate, or sometimes better. Either way, don’t be fooled by the change with the gauges, this could cost you a ticket—lower your indicated speed proportionately and be mindful that the odometer is reading lower, and you should be pleasantly surprised with the higher mileage you obtain. By the way, experiments indicate that the width of the tires does not seem to affect fuel economy

Rubber Internal Friction and Stiffness

Some kinds of rubber are much more “efficient” than others. Remember super-balls? They were so surprising because they could bounce up almost as high as they fell. The ratio of <bounce up> to <fall> is the “coefficient of restitution” of the ball’s rubber. This is the efficiency with which the material returns strain energy as it rebounds. “Internal friction” is a common term for this kind of energy loss. The amount of energy lost by the tire rubber in each rotation is proportional to the amount of flexing, and that in turn is proportional to the weight of the car (for all four wheels combined). This is why rolling resistance is proportional to the car’s weight. These days, many tire companies put additives in the rubber to lower its internal friction.

Now, about stiffness. Remember, I said the weight of the car is held up entirely by the air pressure in the tire? Having stiffer rubber in the tire doesn’t have any practical effect in holding up the car. But, the deflection energy is proportional to stiffness. And a part of that energy is not returned when the rubber rebounds. The effect on mileage is plain to see—tires made of stiff rubber have high Crr, and result in lower miles per gallon.

Let’s look one logical step further while we are still here. Rubber gets softer in hot weather and becomes stiff in the cold. So, we might expect that car fuel economy could increase in hot weather, and this is usually observed to be true. (Engines usually run more efficiently when hot also.) The most efficient energy-saving tires are made of softer rubber, with comparatively flexible sidewalls and tread.

Summary

For best fuel economy from your wheels and tires---

1. Buy only top-quality tires that are known to have high efficiency. The rolling resistance coefficient should be Crr = 0.009 or less. Sporty low-profile tires have higher Crr.

2. Buy tires with a slightly larger diameter, but be sure that they don’t rub other car parts on turns or bouncy roads. Choosing higher-profile tires is a good way to increase diameter.

3. Keep tire pressure higher than normal, up to 10% below sidewall stated pressure, and check tires regularly to be sure the pressure is correct. It’s best to check tire pressure cold. If you think the air temperature will be hotter when driving, subtract one psi of pressure for each 10 degrees hotter.

Suppose you are ready to buy a new set of tires. How do you choose? It’s best if you can know the Crr for the tires. Here is an idea. First, ask if the tires you are buying have a satisfaction guarantee, and how that works. This is partial compensation for not knowing what their “official rolling resistance coefficient” is. Most dealers or tire companies will offer a 30-day satisfaction guarantee. Now, you can drive on them and see how your mileage changes. If you don’t get satisfactory results, you can take them back. You can also measure the rolling resistance coefficient of the tires yourself. That would be a good topic for another lesson.


Ernie Rogers

____________________________

* In the introductory section, the four components of force are spelled out in words. Here is the algebraic equation including all four parts:

F = Cd A 1/2 rho V2 + Crr Mg + M a + Mg • (grade%/100)
The first term, the aerodynamic drag, could be the topic of another lesson.

That's simply too much information for one post

I agree with Drive Stick, but let’s see it I’m on the right page……..

In order to have a lower Crr, that would mean the tire would provide less traction, right. The more it sticks (higher Crr) the harder to get it to turn.

I must have a really low Crr on the Metro now. If we have the slightest snow or ice, I can get stuck on level ground. Yet, on level glare ice, I can push it easier than letting the car do the work!

But on dry payment, we got 53mph a couple weeks ago in March. Now this is Minnesota, so March is not that warm. It was like 45-50 degrees that day. These tires will remain our “summer issue”

I’ve since acquired my “winter issue” tires for next snow season. I’m sure the M&S small truck tires have a much higher Crr, but I should be able to drift bust like nobody’s business!

Kind of a trade off between better FE and spinning your wheels.

I may have missed it, in the above post, but are tires labeled, or coded, for Crr?

Can this be used for non-destruct measurement of relative tire rolling resistance?

Bob Wilson

No kidding, my ADD kicked in early on. Everything ahead of "Tires and Rolling Resistance" can be deleted.

Is that true? :confused:

I suggest reading slower and thinking about what the words say versus just "reading" them. No offense intended; I read it slowly taking about 7 minutes to apply the thoughts to my own experiences. It helps concrete the information in your mind and reference it.

Manufacturers currently do not post that data due to auto makers pushing for them to not. Every tire for every size will have a different Crr, thus making this an extremely time consuming process to measure each, but no more so than load and speed testing. It may also be for safety reasons. If people know the Crr of their tire and get in an accident, lawyers could use that against the manufacturer. Covering their proverbial arses.



I know I learned a few things from this post. Post length has no affect on information digestion. If it helps copy and paste into seperate pages to section the information so the "ADD" doesn't kick in like turbo boost.


EDIT: I copy and pasted into word. just a hair over 6 pages. Damn awesome. You go man.

Hey, wait!! You guys started the discussion without me!!

And if you haven't freaked out by the volume of information Ernie posted, allow me to add:

Barry's Tire Tech

If you read everything on the web site - and not just that page, - you'll find the Ernie and I disagree about calculating the size of the contact patch, and the role air plays in holding up the vehicle, and a few other things that aren't important to the discusssion at hand, so I won't debate those issues here.

But for those who don't want to wade through all that information:

1) Rolling Resistance in tires is mostly caused by Hysteresis - the internal energy consumption of the tire. You could look at this is as internal friction generating heat.

2) Most of the rolling resistance is caused by 3 things: The material properties the tire is made out of, the amount of movement that those materials experience, and the amount of material there is.

3) Most of the energy loss occurs in the tread area, and because most of the tread area is tread rubber, it becomes the most important item in a tire's rolling resistance.

4) The amount of movement that causes this energy loss is mostly controlled by inflation pressure - very little is controlled by the stiffness of the sidewall by itself.

5) Tread rubber chemistry results in a 3 way technology triangle: Treadwear / Traction / Rolling Resistance. Put another wasy: To improve one, you have to sacrifice on of the others (or both).


**********************

OK, that's enough for now. Carry on.

While we are at it, a small query -

The contact patch varies with tyre pressure, as at higher pressures, the vehicle weight is countered by a smaller deformation of the tyres and thus a smaller contact patch.

As the tyres are pumped up to say 50 or 60 PSI (which the auto company did not intend nor prescribe), would its overall dia change to affect the effective turns of tyre per mile and hence the odometer accuracy? What would the delta be from say 30 PSI to 50 (or for that matter 60 PSI) for a vehicle ~2400 lbs?

IMO tread wear should also affect the odometer the same way.

Great post Ernie and everything seems spot on except I think I disagree with the stiffness argument.


Train wheels are made of steel and thus are exceptionally stiff. They use these tires specifically for this reason, since the majority of a train's losses are due to rolling resistance. This stiffness decreases that internal resistance that you speak of.

Those numbers seem appropriate. According to a governmental study every 10% decrease in rolling resistance will result in in a 1-2% increase in fuel economy. Those numbers seem basically inline with that study. The difference between velocities is due to the proportion of rolling resistance and wind resistance when compared to the total energy. Energy lost to rolling resistance is linear and is related to mass, Crr and miles traveled. Wind resistance is not linear and goes up much faster.

Not according to a joint Federal and Californian study on tire rolling resistance. It states they are related, and altering one will generally effect the others, but that it is possible to improve all 3 with out sacrifice. Some tires are better than other tires in all three categories that you mention.

This looks like a good place to pose my question!

I have a stock 2001 Ford Focus ZX3 with tires that are near requiring replacement. I am hoping to improve my fuel mileage a bit when I get new ones. The stock tire size (currently on the vehicle) is 205/50R16. I'm running them at 50 PSI and driving carefully. I usually get between 31-33 MPG for my commute.

My car has the 5 speed manual and turns around 3000 RPM at 70 MPH. It has a high power to weight ratio and I think could use some gear ratio adjustment, with a tire change being the easiest method possible.

I had considered swapping wheels to get a skinnier tire, and also considered just going up on my tire diameter by going with a 205/55, 205/60, or even 205/65R16 size tire. Calculations via this site:

Wheel / tire size calculator / comparer - BIGCUSTOMWHEELS

would indicate that going to a 205/65 would drop RPMs by 9.8% for a given speed, which I would expect to help my fuel economy significantly. I do mostly highway driving, but increasing the moment of inertia with larger tires has me a bit concerned about acceleration losses.

What would be the best move in terms of increasing efficiency by a tire change?

a) upsize the tire modestly (to a 205/55 or 205/60)
b) upsize the maximum (205/65R16, which I think is the largest I could fit in my wheel wells)
c) change to a different wheel--get a skinnier tire on a 14"x5.5" old steel wheel, for example, which would (I think) fit the car

I've read the other threads I could find regarding rolling resistance, and would also appreciate any help I could get in identifying the best tires to get for options A-C above, subject to frugality!

Thanks for any guidance--

Harry

My opinion should be taken with a grain a salt but:

Generally speaking larger diameters tires have lower C_rr according to the government study previously mentioned. It will also improve you gear ratio for highway speeds.

Acceleration losses isn't entirely accurate, its more like deceleration losses. More energy is tied up in the wheels, but it's not lost until you decelerate. If you are efficient about hypermiling your additional losses from the increased MOI should be offset by the other increases.

If you were to ignore the changes to your gearing ratios I would only consider C_rr if you access to the information. Like I said earlier larger tires generally have lower C_rr but the lowest value I have found (and this is an official number) is the Bridgestone B381 with a value of .0065 which is about 35% lower than average. However the tire was a 14 incher if I remember correctly.

Look up the Californian/Federal study on Rolling resistance it is extremely informative. I think the first 40 pages was random crap, but it gets to be very educational later on.


Good luck with your search!

Thanks for the advice.

I had posted about this topic on a Focus-specific enthusiast site, where the general argument was "Bigger tires have a much higher moment of inertia, thus chew up any gains in efficiency from higher gearing by causing you to feed tons more energy into spinning them up." In my reading on this site as well as gassavers.org, I found other data that made me think that the answer probably is to go larger, to a certain point.

I have located some fairly cheap tires ($72-$78 each, free shipping) from savontires.com in the 205/65R16 size and will probably just get 4 of them in the next couple of months. Once I get a tank or two of fuel under my belt, I will post an update regarding my updated MPG ratings.

First, if you read the study very carefully, you'll see that "improve all 3" is carefully worded - and while technically correct it depends on 2 things: That there will be advances in technology (well, duh!) and given the current state of technology, there will be tires that don't use the latest technology (again, duh!)

You should be aware that everyone involved in the study has a stake in perpetuating the situation. Some want to be the group that gets called on to do research - and therefore, they CAN'T give inconvenient conclusions.

Some want to be the guys who do the testing - so they want EVERY tire - that is EVERY SIZE / EVERY DESIGN - tested and their conslusion would be that there is room for improvement.

But the guys who design tires are well aware of the triangle problem. Within what they are allowed to do, they are confronted with this issue on a regular basis - and that's what I was trying to express. Just be aware that while there might be differences between tire manufacturers within each, the triangle applies to each - and at the top rung of the ladder, there won't be much difference.

Just don't expect a tire to give world class rolling resistance, world class levels of traction, and world class treadwear. The technolgy isn't going to allow that to happen.

Blue Bomber pulled the following quote out of Ernie's post, then disagreed with it:

I disagree with Ernie about what is holding the car up, but I think he is correct that rubber stiffness plays little effect on holding up the car.

But I think he is wrong about the effect rubber stiffness has on RR.

1) The steel belts and the polyester ply cords (or whatever they are using) are much more stiff than the surrounding rubber can ever be. Those would play a much larger role in the overall tire stiffness than the rubber itself.

2) Inflation pressure stiffens a tire much more than the tire itself. If you've ever sat on an uninflated tire, you'll know it is easy to deflect the tire. Sit on an inflated tire and there's hardly any movement at all.

That's a double hit for rubber stiffness!

Unfortunately, rolling resistance is a function of something other than stiffness. As I've said before, there is a technology triangle: RR / Traction / Treadwear. And a linear parameter like "stiffness" doesn't quite apply - and I think tread rubber with good hysteretic properties tend to be soft (I am going to check on this, but I am under the impression that good wearing tread rubber tends to be "hard".)

Since there is such a dearth of good tire info out there, I think the bottom line for r.r. is to select the lowest load range tire you can get away with and air it up to max sidewall psi, if not a little more if that's what you want. Oh, and get the most "ribby" (least "blocky) tread design you can find.

Or if you're like me and don't mind using the spare from time to time, run old nearly worn out tires.

I'm with Ernie on this issue of load carrying capacity: Use the largest size tire that will fit in the fenderwells. Larger tires are directionally towards lower coefficients.

No clear connection between Crr and grip
 

Hello, Snowfish,

I don't know that there is a clear connection between tire stickyness and tire efficiency. As I recall, in the greenseal report, one of the tires with the lowest Crr happens to be a winter tire. I thought that might be because winter tire rubber is softer to handle low temperatures. (Just a guess.) And, usually soft rubber means lower Crr.

Ernie Rogers

Measure Crr by bouncing the tire?
 

Wow, Bob, that's a fantastic idea! I can see it! You put the tire on a loose wheel, pump it up to pressure, then measure how high it bounces when you drop in on a smooth concrete surface! Or, see how the bounce changes with the kind of pavement. Lots of possibilities here.

Someone should sit down and work out the math, to see how you convert from bounce to Crr.

I am tied in with the local science fair guy at the high school. I will pass the word. This would be a terrific science fair project. But, don't let me hold back anybody else that wants to experiment.

Ernie Rogers

I get fuel economy from a simple model.
 

Hello, Frank,

That breath-takingly long post-- at the very end, I gave the formula for calculating fuel economy. I set that up for myself in an Excel spreadsheet, and I use it to evaluate car ideas. You can also find this calculating model posted in various places on the internet, but I can't tell you where to look.

The point is, these numbers come out of the car fuel economy model. I have checked the model many times and found it to be reliable. Now, about reducing Crr--it's especially interesting to me that sometimes you CAN cut the rolling resistance in half. Tire Crr varies tremendously with brands, sizes, and tire pressures.

So, yes, it's really true. Smart tire choices make a BIG difference.

Ernie Rogers

Hello, Blue Bomber. You made a good argument. Let's study the question.

Work = force x distance. What we don't want is to put work into the wheel material. For the train wheel, force is a given number, the weight of the train and all its cargo. So, you gain by making the distance as small as you can, and steel wheels don't deflect much at all. (A smart guy willl point out that the rails often deflect a lot, but they spring back and return most of that strain energy.)

Okay, now for the car tire. In this case, it's the distance (related to the contact patch) that is fixed, based on the air pressure in the tire. So, now you want to make the force to flex the rubber as small as possible, and that means to choose material that flexes very easily.

In both cases, materials with low internal friction are the best to use.

Ernie Rogers

I definitely agree with this statement, although I allow that there may be exceptions. (Example: tombstones are routinely cut by sandblasting. rubber is placed over the hard stone where they don't want to cut. The hard stone is cut, but the soft rubber isn't.)

I have a high regard for CapriRacer's knowedge and engineering skill. We can just admit we disagree on a few points now and then. But, notice that this question of how rubber properties and contact patch affect Crr is mostly academic. We should just judge tires by the Crr and forget much of the rest.

My advice to Harry is to use 205/60R16. That was the middle choice for size increase, as I recall. This is expected to give good benefits with much less risk. So, Harry, ask around and make your best guess. I would buy from a local dealer, and I would ask, "do these tires have a customer-satisfaction guarantee?" Most tire companies will let you return tires within 30 days for a full refund. (You may get stuck on the local service, like balancing.)

I would carefully check mileage before changing, and after. And, you can bet I would return tires that don't improve mileage. (I've done it before.) Remember that you must check mileage under identical conditions as much as possible. Adjust speedometer readout downward as needed and use odometer corrections.

Ernie Rogers

I was thinking more along the line of a Rockwell hardness tester approach only using a ball instead of conical tip:
Rockwell hardness testers in portable, bench and automatic digital hardness testers

The idea is to measure the tread material elastic recovery to get an idea of the hysteresis loss. But Michelin did a similar demonstration a year or so ago.

They had two large cars, one with low rolling resistance tires and the other without. They set them on identical "U" tracks in neutral and started them rolling. The low rolling tire car stopped last.

Bob Wilson

Serendipity!
 

Ohhh, okay,

I guess that's how serendipity works.

Ernie Rogers

However if that tire is also wider, you could be trading Crr against CdA:

http://ecomodder.com/forum/showthrea...g-cd-7475.html

And if it's taller (raising the ride height), drag may increase further.

I'm seeing a bunch of theoretical speculation here. Do we have any real world, home brew, life test, experiences where the rubber really hits the road? Not talking test track here. I mean back and forth to work, to the store, the kids softball game, etc.

225/60R17 mpg 26
180/55R14 mpg low 30 high 50(I might have been tailgating a semi. . .:turtle::)

The turtle provides enough low pressure to pull the front of my car. My average is 35.

I used the 17s for a year until they wore out and then swopped in for 14s and new tires.
The 17 tires were hard as a rock. .22 cal had trouble solidly penetrating both sides while most of my other old tires its a non-issue.

Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires

here's one for ernie

Thanks for the article
 

Thank you , Frank,

For showing me that article. It's hard to know for sure what the effect is unless you measure it yourself. I personally did not notice any change on my last tire replacement.

I think the author forgot to mention an important point. When you get the new tires, you will probably drive faster--if you keep the speedometer at the same position as before since the tire diameter is greater. This change in speed would have a greater effect on mileage than the effect from having thicker tread.

Ernie Rogers

New: "Goodyear Assurance Fuel Max" tires
 

Hello,

Has anyone heard about this?

I bought two tires today, from Sears. (Michelin Energy MXV4 Plus-- they were on clearance because they are a discontinued tire.)

Oh, that's not the news The manager said they had new "energy" tires coming from Goodyear, called "Goodyear Assurance Fuel Max" tires. So far, they have to be special ordered but should be in stock soon.

Ernie Rogers

With no easy way to quantify r.r. (no ratings) I hope it is more than a marketing ploy.

Hi Ernie,

Yes, I posted about them a little while ago -- they are also listed in the new TireRack.com catalog; along with a new Michelin HydroEdge Green-X, and Bridgestone Ecopia EP100 Grand Touring Summer LRR tires.

http://ecomodder.com/forum/showthrea...ires-7844.html

Here's a link to the TireRack page for the Goodyears:

http://www.tirerack.com/tires/tires....rance+Fuel+Max

We need a normalized tire rating
 

Hello, Frank,

You have put your thumb on the big issue the tire makers have to find an answer to.

Here is an idea. Any tire makers listening? Test all tires at the same (standard) spring rate, by adjusting pressure. This will help to simplify the situation.

The next possible simplification could be to all agree to publish the rolling resistance coefficient for certain specified sizes that are common-- for example, one size for compacts, another size for SUVs.

What do you think?

Ernie Rogers

If there was any truth to this then why do steel belted radial tires have a lower rolling resistance than than an ordinary radial tire?

Also note that the soft or hardness of a rubber has nothing to do with its internal friction.

Steel can be flexible when it is thin. #0000 steel wool feels soft, and burns in air if a spark gets on it.

The steel strands used in a steel belted tire is considerably thicker than that used in steel wool. Steel belts are used as stiffeners in tires to make the tread inflexible. This lowers the rolling resistance.

Actually, the main benefit is not stiffness per se, but the directions of stiffness and flexibility. The belted tire tread squirms less as it conforms to the road.

The stiffness however contributes to the low rolling resistance.

I said that soft rubber gives lower rolling resistance.

Then, Clark said,

Well, I think I will start with a disclaimer-- While I have designed many structures over the years, I have never designed a tire. I will be giving my impressions, based on engineering principles and sometimes I might get it wrong. Now, here goes.

In my view, the pneumatic tire is one of the greatest inventions of all time. (Along with bar soap.) Here's why an air-filled tire is such a great invention. First, having some give in the tire is essential to protecting other car parts from jarring loads. Essentially, we want the tire to act as a spring. There are lots of springy materials a person could use in a tire, and they all have a weakness, except one. The weakness is that when you flex a solid material, you never get all the energy back when you let it unflex. Air is the wonderful exception. When you compress air in a short time (adiabatically-no heat loss), and then let it go back, no energy is lost. So, except for whatever energy is lost in the tire shell, a pneumatic tire has perfect efficiency.

Now, let's deal with the shell. The ideal shell material holds the air pressure but has the least involvement in the spring action. It needs to have good flexibiliy, no stretch in the tread area (but flexible), and the right give in the side walls (with minimum stretch) to form the contact patch.

Bending is the desired type of shell deformation. When a sheet of material bends, energy is absorbed as strain energy, by a formula like this--

S = a K T^4

Which says that the strain energy S is proportional to the stiffness K times the thinkness of the material raised to the fourth power. Each time the material flexes, energy is lost. The energy returned is proportional to the coefficient of restitution, let's call it R, and for the energy lost, it's 1-R:

Energy lost = (1-R) S

You should notice that the energy lost is proportional to the stiffness, K. In some ways at least, the thickness of the shell is set by the strength needed to contain the pressure. Steel wire in the tire provides great strength, allowing the shell to be much thinner and thereby reduce the strain energy in the rubber, which is where the energy loss occurs. The steel cord has very low internal friction (1-R) and lies on the neutral axis in bending, so almost no energy is lost in the steel.

So, contrary to intuition, steel cord has the effect of improving flexibility in the rubber layers and reducing stretch, and thus improves efficiency.

Now, as to whether or not hardness (stiffness) of the rubber is related to the internal friction, I admit there is some uncertainty there. But, the formula directly above suggests that the energy lost is proportional to the strain energy (for a given value of 1-R) and that in turn was proportional to the stiffness. It seems that silica is often added to the rubber to lower internal friction, and I'm sure that doesn't make the rubber softer.

Ernie Rogers

Nope, I'm gonna have to go with liquid soap being greater.