Going from cheapy tires to Star Specs also knocked about 3 MPG off my average, just like Rob-2.

Tires molecularly bond with the road (this is one part of the traction equation, the other being interlocking irregularities), and softer ("stickier") tires do more bonding than hard tires. The rubber resists being pulled off the pavement.

That same molecular bond is also the general factor that results in things like wider tires gripping better than narrow ones, reduced body roll improving overall grip, the effect of changing one axle's sway bar on balance, etc. I won't bother explaining why as it would take forever.

Skip Barber's "Going Faster" is a great book that explains it all much better than I ever could.

 

I'm just curious why you would want to reduce the rolling resistance. Is this specifically to improve your gas mileage?

I also have Star Specs but have only noticed an increase in grip, I haven't really noticed a change in mileage.

 

Still not buying into the "molecular bonding" factor as it relates to rolling resistance on street tires on the street. It's not like the tires are so sticky that they're having to be peeled off the road as the car rolls along. Any "molecular bonding" or tire adhesion is going to be more related to lateral/longitudinal tractive grip and slip angles required to get that grip. A "sticky" street tire like the Star Spec simply rolling along without having to generate grip shouldn't be giving much if any greater resistance to rolling than other less grippy tires of similar size/weight.

I'm getting mileage with the Star Specs comparable to what I was getting with the winter tires that were on it a few weeks back. Maybe 1mpg less, but the last few tankfuls have been at track inflation pressures of 28psi rear/30psi front instead of my usual street pressures of 32-34psi cold all around. Last tank got me 260.5 miles on 9.2 gallons, just over 28mpg. I'll report back how this tank does in a couple of days, now that I've re-inflated the tires to street pressures again.

 

look at tire pressure, alignment, and any extra baggage you may be carrying in the car.

 

You don't need to "buy into" it, that's actually exactly what happens and how grip is produced at a molecular level.

 

But we're not *talking* about grip, we're talking about rolling resistance.

My experience: "Sticky" tires don't necessarily have more rolling resistance or reduce fuel economy.

 

I'll grant this:
With significant toe-in, stickier gripper tires will generate more lateral grip while driving straight ahead (lateral forces from tire or tires on one side of the car equally opposed by lateral forces generated by tire or tires on the other side), which would act to scrub off some mpg.

 

In all cases there is some amount of lateral movement of the tire (molecularly or not) even when traveling straight. Especially as the tire bends coming into contact patch and bends coming out of contact patch. Which, on a more "sticky" tire, always equals more rolling resistance.

I also switched from kumho ASXs to Dunlop Stars and saw a drop from 28-32 to 25-30.....I just replaced my air filter so hopefully that'll gain a little something (it was pretty rough ha ha).

Alignment and increased air will definitely show a difference.

 

[QUOTE=ZDan,May 18 2010, 01:47 PM] Still not buying into the "molecular bonding" factor as it relates to rolling resistance on street tires on the street.

 

Rolling resistance will have a lot more to do with tire mass and internal construction than with compound.
[compound will be a factor, but not due to "stickiness" but rather due to it's ability to be deformed and rebound without losses.]

"Stickiness", particularly of a street tire being driven on the street (as opposed to R-compound tires at temp on the track, although I'd bet money the "stickiness" factor as it relates to rolling resistance is pretty damn small there as well), isn't going to amount to much as far as rolling resistance goes, the other factors mentioned and tire pressure will dominate.