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Old 04-10-2009, 09:30 AM   #11 (permalink)
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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

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Old 04-10-2009, 06:15 PM   #12 (permalink)
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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.


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Old 04-11-2009, 08:08 AM   #13 (permalink)
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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.
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Old 04-11-2009, 08:33 AM   #14 (permalink)
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Quote:
Originally Posted by Blue Bomber Man View Post
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.
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.
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Old 04-11-2009, 09:06 AM   #15 (permalink)
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Blue Bomber pulled the following quote out of Ernie's post, then disagreed with it:

Quote:
Originally Posted by Ernie Rogers View Post
.....

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.

........
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".)
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Old 04-11-2009, 04:49 PM   #16 (permalink)
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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.
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Old 04-12-2009, 09:02 AM   #17 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
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.
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Old 04-13-2009, 03:28 PM   #18 (permalink)
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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

Quote:
Originally Posted by snowfish View 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?
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Old 04-13-2009, 03:36 PM   #19 (permalink)
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Measure Crr by bouncing the tire?

Quote:
Originally Posted by bwilson4web View Post
Can this be used for non-destruct measurement of relative tire rolling resistance?

Bob Wilson
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
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Old 04-13-2009, 03:58 PM   #20 (permalink)
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I get fuel economy from a simple model.

Quote:
Originally Posted by Frank Lee View Post
"..Cutting Crr in half causes—
...at 60 mph: 14% better mileage
...at 40 mph: 20% better mileage."
Is that true?
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

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