[IndyIan]

Dave,
I think in practice you are right, bigger diameter tires on trucks leads to worse mpg.
But I think its because; the bigger tires are almost always wider, have more rolling resistance due to off road tread patterns, and are heavier which hurts city mpg.

I have 205/75R15's(minivan tires) on my Tracker, and if I could find something like 185/95R15's in an all season tread pattern I think I would do better on the highway with the 6% reduction in rpm. It would be about a 200rpm difference which would get me closer to 2500 rpm at 60 mph.

Last Sunday I had sticky high performance 195/45R14's on the back of my Neon for autocross which lowered the back end about an inch, but I think even just 2 increased the rolling resistence compared to the normal 185/65R14's. I drove with the normal tires on the front and put my R compound tires on the front at the track. I wasn't getting my normal coast distances and the tires were noiser than my all-season tires. Also I think any misalignment creates more drag with low profile tires than high profile tires.

Probably the best bet for increased FE is go with the narrowest, highest profile tires you can find that fit on your stock rims. Usually this won't be very much bigger in diameter for most cars.

Terry,
Aren't 13" tires half as much as 12" tires? and there is way more selection? I would go with 13's just for those reasons, plus 1800 rpms is better than 2000 rpms at 55 mph and you don't have to change gearing at all.

Ian