Tire physics Q: new CR-V tires, major gas mileage hit

I just replaced the OEM Bridgestone Dueler H/T D684s on my 2000 CR-V with
identical sized Pirelli Scorpion STRs. The Pirelli's have much better UTQG
ratings for temperature and more than 3x the tread life rating. (180BB vs.
520AA.) So far they seem like better tires in every regard (noise, road
feel, handling, snow and ice, etc.) but one: the gas mileage has gone down
by a bunch. (A bunch is more than 10% and maybe as much as 20%. Four tanks
since getting the tires are 20.5 MPG average and 12 before were 25.2
average--there are other reasons like arrival of winter weather and
different non-standard trips factored in there that make me a little
suspicious that this might be worse than the problem really is.)

This is not a one fill fluke, and I think it's real from driving
characteristics: I seem to have to use more accelerator to do the same speed
and on hills I travel frequently, the vehicle seems to spend a lot more time
downshifted. I have checked and re-checked the tire pressure. I've even run
them higher than the door jamb pressure by 3psi (which really hurts ride
quality but seems to make no difference in the basic problem.)

So my question is this: where's the energy from the apparently significantly
higher rolling resistance getting dissipated? Based on the UTQGs, it
shouldn't be in heat buildup or tread wear. What's left?

 

Could be a combination of things. More flexible tread that heats more
at speed. Maybe the brakes aren't releasing as fully after the tire
work. My real guess is that the tires are slightly larger and you are
going slightly faster and farther than you think.

Good Luck,
Roadie Roger

 

Thanks for the reply. The size difference is an interesting thought I hadn't
really considered. Given the magnitude of the *apparent* difference in
performance/mileage, it seems hard to believe that the new tires are that
much larger, but I certainly haven't tried a calibration run. I guess I was
rejecting increased heating at speed given the huge improvement in
temperature/tread life UTQGs. Of course, I've long sensed those numbers were
cooked anyway.

 

Aren't treadwear numbers on UTQGs only applicable within the same
tire company? I.e. a Bridgestone 200 isn't necessarily the same as
a Pirelli 200 in treadwear?

Check your tire pressure as well. Low tire pressure can reduce fuel
economy.