Tires: Sidewall stiffness and fuel economy

[CapriRacer]

Originally Posted by RPM

This is slightly tangential but it seemed better to bring this thread back instead of creating a new one.

My question is this: is there a relationship between sidewall size and rolling resistance?

I.e., all else being equal, would a 195/50 R15 tire have a lower rolling resistance than a 195/60 R15 tire?

The theory being that a smaller sidewall deflects less and since deflection is what causes rolling resistance...

First let's start with "All other things being equal". That is really really hard to do. Change tire size, change load carrying capacity, change diameter, etc.

But let's start with this:

Barry's Tire Tech

The formula for the relative RRC by tire size says the difference between a 195/60R15 and a 195/50R15 is only 3.3%, where the difference within tire size is 60%.

So in the big scheme of things, a 60% difference is so much bigger than a 3% difference it is hardly worth the effort.

[RPM]

Thank you Barry, I love your site and was reading it before I posted my question.
The data you used in your regression doesn't include any low profile tires so I was wondering whether you could apply the formula in this case.

[CapriRacer]

Originally Posted by RPM

Thank you Barry, I love your site and was reading it before I posted my question.
The data you used in your regression doesn't include any low profile tires so I was wondering whether you could apply the formula in this case.

The formula was derived from the data available. The data did not include lower aspect ratio sizes, but "if everything else were equal", there is no reason to believe the trends shown in the data would change the formula. It would be nice to find some confirming data, or data which expands the range.

There is however an odd thing I should point out - and it's the "Everything else being equal". Lower aspect ratios tend to use higher speed ratings. For example, Tire Rack lists in a 195/60R15 size, 1 S, 6 T, 25 H, and 2 V speed ratings. In a 195/50R15: 1 T, 1 H, and 7 V speed ratings.

I can argue that, more or less, all S and T rated tires are built the same. But I can NOT argue that H rated tires are the same as the S's and T's - and the V's are another thing again.

That difference in construction affects RR - higher speed ratings having more material would have higher RR values - PLUS - higher speed rated tires also tend to have higher grip levels, by sacrificing treadwear and RR.

So that "Everything else being equal" creates a situation that doesn't reflect reality.

[RPM]

You are right, in reality it's really hard to isolate all the variables. In practice the choice for my car is between: 175/65 R14 H, 185/60 R14 H and 195/50 R15 V. Currently I have the latter, which is best for handling.

I read it on your site and it is interesting to me that high speed rated tires tend to have higher rolling resistance coefficients. I was hoping that by deflecting less they would roll better.

The same could be said for side wall size. I was hoping a short side wall would help, but your regression shows the opposite.

[samwichse]

You could always throw the 165/65 R14 re92 Potenza on there. It's pretty much THE eco-tire, although it turns about %5 more RPMs/mile than a 175/65/R14, I'd bet you'd still come out ahead on the MPG.

Obviously rim-width dependent, I think the widest rim it'll fit is 6"

Sam

[RPM]

I can't. Where I live the tire size must match one of the ones listed in the car documents, otherwise it will fail the annual safety inspection and be subject to a fine. Those 3 are the sizes I can use.

ps: the amount of leeway you guys have in the US is incredible. I once got the car impounded just by having a front strut bar installed, which was actually a Honda part, just from a different variant of the same model car… Had to spend a huge amount of money to get an automotive engineer to write a project and then submit it to the authorities for homologation. Conspicuously modded cars like many on this site would never work here.

[cosmick]

"Plus" fitments, if done with forged aluminum wheels, reduce the weight of eavh wheel/tire assembly, which helps MPG, especially when accelerating, and also helps brake cooling. But it looks bad unless you add larger brakes, which is a net improvement in both performance and safety, but is a net loss of MPG.

[cosmick]

Originally Posted by RPM

Where I live the tire size must match one of the ones listed in the car documents, otherwise it will fail the annual safety inspection and be subject to a fine. Those 3 are the sizes I can use.

So use the OE junk to pass inspection, then run what you like the other 364 days every year. Not like the police can do anything more than make you go get re-inspected, for which you can fit the OE junk.

[cosmick]

Originally Posted by samwichse

You could always throw the 165/65 R14 re92 Potenza on there. It's pretty much THE eco-tire, although it turns about %5 more RPMs/mile than a 175/65/R14, I'd bet you'd still come out ahead on the MPG.

Obviously rim-width dependent, I think the widest rim it'll fit is 6"

Sam

As you increase wheel diameter, this often lets you add positive offset to the wheels. This may reduce turning radius if you don't clearance the suspension, but is worth looking into. For example, '82-'92 Camaros can only fit 8s in a 16" size, 17s can fit 9.5s, but 18s can fit 11s, all without sticking out from under the stock fenders.

[RPM]

Originally Posted by cosmick

So use the OE junk to pass inspection, then run what you like the other 364 days every year. Not like the police can do anything more than make you go get re-inspected, for which you can fit the OE junk.

No, the police will give you a ticket on the spot if they catch you. And unlike the US where the police will only stop you if they suspect something, here we have random routine police inspections on the side of the road.

No way. It's not worth the risk.