Tires: Sidewall stiffness and fuel economy

[AeroModder]

I'm looking at tires to get that'll lower my rolling resistance and give me better gas mileage, but I'm also looking for something that isn't squishy in turns. I've been looking for a tire with stiffer sidewalls than what I have now, and I just started comparing max inflation pressure. I was looking at the Pirelli P4s (Pirelli P4 Four Seasons), but I noticed the inflation pressure is pretty low. Now I found the Yokohama Avid Touring (Yokohama AVID TOURING-S) which has a slightly lower treadwear rating, but can go up to 51 PSI. To get the stiffer sidewalls, I'd need to get 195 wide tires instead of the stock 185, and I know the wider tire takes a hit in drag, but the 185s are all softer-walled.

Has anyone noticed if the sidewall stiffness has any effect on fuel economy?





EDIT: After some reading, I'm finding completely contradictory reports. Some say a stiffer sidewall rolls easier, reducing RR, but other say that a softer sidewall absorbs bumps better, reducing RR. Which is it? Soft or stiff?!?

[CapriRacer]

The best information I have been able to find is that sidewall stiffness - that is the stiffness of the sidewall by itself - plays little role in rolling resistance.

However, inflation pressure does. Inflation pressure will stiffen a sidewall and in fact is the primary component when you want to compare sidewall stiffness inflated. For that reason, ride quality is very much tied to inflation pressure and only slightly so to the actual stiffness of the sidewall by itself. That is not to say you can not feel the difference in stiffness between tires. It is to say that inflation pressure has more of a difference.

[NeilBlanchard]

I am wrestling with this decision right now, too. I think the part that helps lower the rolling resistance is the construction of the tread. If the tread remains closer to round, then the rolling resistance is lower. The sidewall matters if it contributes to the outside tread remaining closer to round -- if it flexes all around the tire, rather than just at the bottom point of contact, then I think it is important.

But, even the higher sidewall max is not an indicator of lower rolling resistance: the Michelin Energy Saver A/S is (apparently) the lowest rolling resistance, and it has a sidewall max of 44psi.

[AeroModder]

I was thinking the higher max inflation pressure would be safer to run at 50 PSI.

An article I found last night explained that stiffer sidewalls can reduce rolling resistance by reducing the deformation, but the increased weight adds RR. The Pirellis touted as LRR tires are 3 pounds heavier than the Yokos. I'm thinking if I go with the Yokos and inflate them up to 50 PSI, I'll be able to match the RR of the Pirellis. Either way, I'm sure they'll be better than the Douglas XtraTrac IIs I'm running. The aggressive tread pattern can't be good for airflow, plus they're garbage in the rain:


I got them thinking that they might help with snow traction and be a bit grippier than the Goodyear Viva 2s. That, and I needed the cheapest rubbers I could find to get me rolling. After reading some reports about the steel belts separating after 30k miles or less, I think I'm going to replace them. I already put 10k on these.

Now, it's to find a new set. Some people are saying the Yokos got them a 1-2% increase in MPG, and that's with 35 PSI. I wonder what 50 will do? XD

[CapriRacer]

Here's the long version:

Barry's Tire Tech

Here's the short version:

I don't think tread "roundness" gives you much indication of the rolling resistance of a tire. But I think the UTQG treadwear rating does - lower UTQG treadwear rating = better RR.

But beware when folks claim their fuel economy went down after replacing tires. More tread rubber = more RR, so wornout tires have better fuel economy, all other things being equal.

[gone-ot]

...think "new tire" RR being akin to LONG rubber stalks (tread):

A) the LONGER they are (new tires) the more they can bend as they rotate into contact with the road; while...

B) the SHORTER they are (worn tire) the less they bend as they rotate into contact with the road.

...and, it's the bending of those rubber stalks (tread) that's the energy hystersis that's a major component of RR.

[AeroModder]

Originally Posted by CapriRacer

But I think the UTQG treadwear rating does - lower UTQG treadwear rating = better RR.

I don't think that is exactly true. Most sticky performance tires have low treadwear ratings, and also high rolling resistance. My current tires have a rating of 420, and they don't seem to have low rolling resistance.

Found a Prius driver's comparison between the OE Goodyear Integrity, BFG Touring T/A Pro, and the Yokohamas I'm looking at, the Touring S.

http://priuschat.com/forums/gen-ii-p...touring-s.html

The reviewer says the Yokos got him 1 MPG more than the OE Integritys, which are listed as LRR tires. Looks like I found my new rubbers.

Now the question of sidewall stiffness remains. I'm very tempted to go with the 195 wide, since the load rating for those is 90, and the 185 wide has an 87 rating. My current ones are 88 and feel a bit squishy in turns and sluggish to respond to steering imput. I'll most likely be going with the 195s unless I find that having the softer sidewall will net me at least 2 MPGs more. I'm willing to make a small tradeoff to have a better handling tire.

[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...

[Chrysler kid]

Yes and no, your speedometer would read slightly higher

tire width and pressure has more of a difference. I believe a taller tire would take more psi at max however that is offset by the tire being taller. So more max sidewall psi but the same contact area

My truck has 80psi max sidewall psi because they are so tall

[RPM]

Thanks for the reply! Please ignore all factors other than the sidewall height difference (i.e. assume same overall diameter, tread compound, tire pressure, etc).