Size of the tire - are smaller (narrower) tires really better?
 

Hi,

i have an question which i have a long time in my head:

Often ist said that smaller tires (not smaller amount) are better to save gas.

But i read in a forum for racingbikes (cycle) that tires which are very narrow have a higher rolling resistance than wider tires at same inflation. This is because of the friction inside the small tire. A wider tire has less deformation at same pressure as a narrow tire.

In wikipedia i read that a tire of a normal car has a rolling resistance (Cr) of 0,011–0,015, ad a much more bigger tire of a big truck (LKW) has only 0,006–0,010 on same track.

I´m planning to buy new summer tires with rims for my golf, and can´t decide between 175/80R14 and 195/65R15. With this information the 195/65R15 should have a smaller rolling resistance, because there is more air inside, and with its bigger wide the deformation should be less than in the narrow tire with high rubber.

Has anyone some information about this things? Could it be that the most narrow tire isn´t the best in rolling resistance?

The diameter of my tires has to be the same because of german laws and police...

Kind regards,

Benny

For as much as i know ussually the smaller width tire the better FE, one reason why companies throw small width tires on bigger vehicles to help with the mileage.
Ex. my brothers 01 grand cherokees tires are about the same width as my grand ams. Although the cherokees are trail tested it isn't doing much with small tires on it, but save gas

You need to define 'smaller'

What you mean is narrower. Smaller means smaller diameter.

Smaller would prove for worse mileage.
Narrower would provide such a small gain that I would not concider the lose of handling and comfort.
Have you aired up the tires to 40-45psi?

I cant even find a 175/80/14 on tirerack(largest seller of tires)

you want to look at a tire w/ an LRR rating (low rolling resistance)

you also dont want to go below the weight rating.

Fundamentally, there is relatively no difference in a slightly narrower tire, and you would not want a REALLY narrow tire. Not safe.

I would be very slow to use narrower tires than came on the car. The narrower tires compromise braking, cornering and transient handling (accident avoidance). The tires are your connection to the road, the tires are the single biggest factor determining whether the car does what you tell it to. Tires are no place to skimp.

I'd run skinnier tires for sure. Has anyone that is scared of skinny tires actually tried it???

That said, CapriRacer says skinnier tires generally don't reduce rolling resistance. I think it's the kind of thing that varies on a case-by-case basis; I can't just say "skinny tires will reduce r.r." or "skinny tires will increase r.r." but in general my opinion is IF you are not loading the vehicle to it's capacity very often or not at all (like I do) a skinnier than stock tire could very likely help.

That size might be commonly available in Europe. I've not searched to confirm but I think Vekke has done some work in this area.

Link to more information on his site on this subject:

Barry's Tire Tech

and

Barry's Tire Tech

I like size appropriate, some cars come with too wide of tire for their wieght IMO. My 3000 lb malibu is better suited for 205 probably 195 in the winter or rain than the factory 215. Cobalt is 400 lbs lighter with 195, escorts and sentra's I've owned about same weight as the Cobalt had 155's and they cut thru the snow great. Barry say's wider are better and I believe it, but it's a compromise between traction and rolling.

If going summer and winter, go wide summer, narrow winter.

the most i can say on the matter is to suggest anyone contemplating buying a different than stock size consider what that means about the pressure on the contact patch.

the smaller diameter means you have more pressure on the contact patch, because the contact patch will inherently shrink from front to back (think higher angle of attack). if you go narrower instead, you also increase the pressure on the contact patch, but at a faster rate. if you go smaller and narrower, you get much higher psi on the contact patch.

i wont guess at what this means to traction, handling, etc... BUT, the rubber that tires are made of can only handle a certain pressure before it deforms too much, just something to keep in mind. small tires + heavy car = bad.

never thought about it until my met 245 class when we talked about rolling metals. instinct makes you think they would use a large diameter roller, but they actually use a small diameter roller that is actually in contact with the metal, because it has a smaller contact patch and requires less pressure. also there is a phenomenon where once metal starts to deform it deforms rather easily if done fast enough.

The area of the contact patch at the same pressure remains the same, whatever the size of the tire. In other words, traction and handling should be unaffected... all else being equal... of course, sidewall stiffness is a much bigger issue with a narrower tire... and narrow tires don't come with the nice sticky rubber you can buy with wider performance tires, but if you're getting the same exact tire model in different widths, the only thing working against the narrower one is the flex in the sidewalls.

What changes there are are in the shape. Narrow and small are not ideal because the shape is long instead of wide. This means more flexion of the tire and more heat build-up, with less tire to absorb it... which is not ideal.

Sorry, my fault. I meant narrower tires, with same diameter.

Both tires are stock recommended sizes. 175/80R14 is the narrowest regular size for this car, 195/65R15 is the most sold tire in germany, and also stock recommended. In size 175 there are only an few different tires to buy, but in 195/65R15 there are about 300 different profiles and a lot of LRR-tires to buy.

In the moment i´m driving very bad all-weather tires from the last owner of my car. So i need to buy some new tires and rims for summer.

I´ll drive the tires at maximum sidewall pressure, but actually the mounted tires are so bad, i wont try it with them...

Kind regards

Benny

Bigger is better!
 

The best information I have been able to dig up is that bigger is better - no matter how you define "Bigger".

That said, the effect is small - especially compared to the differences between tires.

In other words, spend your time researching tires and very little researching tire size - but if you have the opportunity, go larger.

Not a direct comparison. However my 95 accord with 185/70/14s and lowed CoD does not roll with EOC nearly as well as my 98 Cavalier with a high CoD and 195/65/15s. Both are around 50psi in the tires. Also, my accord is a death trap in the rain with those skinny tires and no ABS.

wasn't there some one that tested the idea of using 4 space saver tires on a auto and lost some mpg.

Oh please dont bring that back..:eek:...It didnt work/wont work.

I compared the rolling resistance of 4 space saver donut spares on the car against various other tires in my fleet of various sizes. The donuts were the worst of the lot: http://ecomodder.com/forum/showthrea...uts-19094.html

http://forkenswift.com/album/8-results-chart.gif

from: http://ecomodder.com/forum/showthrea...e92-19126.html

Barry/Capri, can you please help me understand the coefficent of RR versus the RR force?

To me it seems that we have a fixed weight (to a close approximation) of vehicle, so we would be putting a fixed load on a tire. I would also assume (and it may be a bad assumption) the same inflation pressures in each case. And for comparison we also need to assume the same construction, rubber compounds, etc. In that case, the lower RR force for narrower tires would seem to mean that those tires would coast farther.

Why is it the coefficient, which you have to multiply by the load, which actually shows you which tires would coast further?

-soD

Rolling Resistance Force (RRF) = Rolling Resistance Coefficient (RRC) X load on tire.

When tires are tested for RR, they are tested at a particular load and inflation pressure. The result is a Force. If you divide by the test load, you get a dimensionless number (a coefficient) that applies to that particular test fixture and that particular test procedure at that particular inflation pressure.

If you want to compare tires, you need to run them all at the same time, at the same place, and under the same conditions.

Questions:

1) Do the values vary according to the test fixture? Yes, but a correlation between test fixtures (and therefore test facilities) could be done so they could be compared. I'm not going to go into what it would take for that to happen but it is considerable!

2) Do RR values vary according to the test method? Yes, but a correlation between tests can also be done. Again a considerable coordinated effort would need to take place.

3) Is RRC constant for a given tire? No. Not only does it vary according to inflation pressure, but it varies according to load. The best information I have is the variation due to load is small enough to ignore (but it is something to keep in mind)

Coming back to the same tire in different sizes, I think the reason the data shows that "Bigger is Better" is that an increase in tire width of (say) 10mm increases the width of the tread a fraction of that (say 7 mm), but the increase in load carrying capacity is pretty close to the increase in width. The net effect is that there is slightly less material in comparison to the load.

And did I mention that while the sidewalls only play a small role in RR, every tire only has 2. Changing the tire size doesn't change how many of them there are.

I think what is being lost here are the effects of rolling mass and aero.

So when driving down the road which trumps which
Rolling Mass + Aero or Rolling resistance by itself.

Obviously crap tires made of bias crap compounds won't score well.

Short answer: it depends. ;)

You can find a good discussion of aero losses vs tire width in this thread: http://ecomodder.com/forum/showthrea...g-cd-7475.html

You can also use the aero/rolling resistance calculator to estimate effects of changes to Cd/A & RRC from tire changes: http://ecomodder.com/forum/tool-aero...resistance.php

If your tread patterns were similar, narrower will typically work better in the rain and snow as there is more weight per unit area, displacing the water more effectively.

Perhaps it's because I'm unreasonably dense, but isn't the Force the thing that directly affects coasting?

Are you saying that we can never assume that the load is the same from tire to tire, even on the same car and if we set the same pressures?

I think, that if I am looking for tires to put on one specific car (e.g., my own car) that if I am allowed to set whatever tire pressure I want, the load would be the same no matter what the specific tire was. So in that case, the tire with the lowest measured force resisting movement would be the best one for efficiency.

Can you please show me where I am getting this wrong?

-soD

^^^ That sounds correct. The business about a particular load and inflation pressure is to gain the ability to compare tires directly, independent of the application.

You're right - it's the RRforce that actually causes the drag and needs to be overcome to move the vehicle.

Let me put it like this:

- and for the purposes of discussion and simplicity, I am going to confine this to passenger car tires - standard load only.

When tires are tested for RR, the test method will describe the testing conditions. Typically the pressure is specified (remember, we are talking about SL PC tires!) - say 30 psi.

What is also specified is the load. Since the rated load on a tire varies by tire size, this is usually expressed as a percent of some value in the load table. Larger tires will have more load than smaller tires, but the way it is calculated would be the same.

So when you try to compare tires of difference size, since they were tested at different loads, the RRF values are NOT comparable. The only way to compare tires of different size is by RRC - which takes the load out of the picture.

You should also be aware that RRF varies by inflation pressure - and to my knowledge, no one has developed a formula that covers every tire to account for changes in inflation pressure. Put a different way, there isn't a standard way of comparing tires tested with different inflation pressures.

Compared to the weight of the vehicle, the mass of tires is a tiny fraction - it's impact would be minimal.

The difference in inertia between tires is also fairly small - especially when you consider the inertia of other rotating components.

Aero? The difference in aero drag due to tire width is also pretty small - and I've been able to demonstrate that the difference in RR due to width more than offsets the change in aero.

Bottomline: You can neglect everything else if you want to talk about tire size and its affect on RR. But you also have to consider that tire size has a small affect compared to the difference between tires (meaning make and model). Spend your time worried about tire make and model and not so much about tire size.

So what you're saying is that tires of different sizes are being loaded with different loads (as expressed in lbs of force), because those are the same percentage of the load value in a table somewhere.

OK, that makes sense of the results--one of the assumptions I was making was invalid, so my reasoning fails at that point.



True, but a limited number of examples tested have shown us that RRF drops as pressures rise. The relationship is non-linear to be sure, but it does seem to be monotonic in the range from typical placard pressures to the sidewall pressure. It's definitely not a complete model by any means though, and it is quite possible that there are exceptions.


Anyway, thanks for clearing up my understanding of the data!

-soD

Its true the real tests we have all show slightly bigger is almost always better.
I went to a slightly wider and 2 inch taller tire on the suburban I was fully expecting fuel milage to go down, but it stayed the same and may have even gone up slightly.

Wasn't there a test were some one on here with a metro put bigger tires on the front (where all the weight is) and doughnut tires on the back and picked up like 4mpg?

I run donuts on all 4's of my 1980 commuta 125r13's and my rolling distance is nearly double that of a legitimate 155r13 (but I am only using a very small percentage of the donut tires load rating)

Cheers
Ryan

If you can pull those from the junk yard new, all day for $10 or $20 each, its unbeatable.

Half were $5 the other half were $10 and I get a few cents for the rims. :)

Best part is they are the correct size for the car.

From my experience yes, less weight and size means less rolling resistance. A simple formula is the max load of the tire should be equal to or greater than half of the GAWR front or rear on the Vehicle Certification Label, usually the rear is less on a fwd car. From there the smallest wheel with the smallest tire is best. You can easily find tire specs on tirerack.com This should be done before lowering the car as it may make it unnecessary to do. There may be a limit to how small of a wheel you can use to fit over brake calipers.

cargo,
can you show us your tests that verify this.
it runs contrary to my tests and those of others.

smaller DIAMETER of tire results in HIGHER RPM at cruise speed. Higher rpms result in lower gas mileage.

The test that I used was the simple fill up to first stop and then use a full tank of gas from a 0 trip odometer and then fill the same way and then divide the trip odometer by the gallons on the pump. This was on a Nissan Versa 4D 1.8SL CVT(which helped) going from a 225/45/17 (summer tires) to a 195/55/15 (snow tires) and consistently showed a difference of 3mpg exactly, 30 to 27 respectively. Even with a difference of 1.6" on a non CVT car you're only talking a difference of a couple hundred rpm which is negligible.

Here's a calculator you may want to make a sticky

Tire Size, RPM, Speed, and Differential Ratio Calculator

So was the difference in tire size factored in (6%)? The other 4% could easily be differences in rolling resistance differences of 2 very different tires

It is REALLY important when doing tests (A-B-A ideally) that the variables are consistent.
1. The thread is about TIRE size. It is important to understand that it means ONLY tire size. 15 to 15 etc.
2. Tests comparing different size(14, 15, 16 etc) tires means you have different rims. so weight of each rim HAS to be taken into consideration.
3. Since the rims are different sizes, then there is the consideration for the movement of MASS in and out.
4. Finally, summer tire to winter tire has some weight difference, especially on different size tires and compound difference.
5. finally, you don't say if you are factoring for the odometer being off.:confused: YOu seem pretty clear that you don't. THe best way is with GPS mileage, it's always right regardless of tire size.:thumbup:

Cant really say that you 'tested' the results.

Again, MULTIPLE members have posted true tested results. A smaller TIRE, doesnot produce better mileage.

I wouldn't bother going with a smaller tire unless I was going to a smaller rim. Again the speedo error would be negligible and not in favor of the smaller tire anyway recording more miles, so yeah I was probably getting better than 3mpg improvement.

Here's another calculator for you:

http://tire-size-conversion.com/spee...r-calibration/

Yes, the smaller tire would record more miles, about 6.1% more miles.
So if you travel 100 actual miles, and use 2 gallons of fuel, you would figure your mileage as 50mpg. With the smaller tires your odometer would show 106.1miles, even though you had only traveled 100 miles, and if you consumed the same 2 gallons, you would falsely calculate 53.05mpg

A smaller circumference will reduce both speed and distance covered so to compensate you'd need to drive both faster and further, at least according to the speedo/odo.
Using GPS data is indeed the best way to resolve it.

Then your winter tires are softer, have higher side walls and narrower thread, these things will contribute to the FE. Even at the correct speed an distance those may compensate for the higher RPMs and higher rolling resistance from the shorter tire radius.

With so many things to consider it is near impossible to draw conclusions from this test alone.