yup. with my 3.73's, i went from the stock size 235/85R16, firestone transforce A/T's to 265/75R16 general grabbers and lost a good 1 MPG on average.

or, put it this way. i drove to NY from my home in june 08 on the firestone 235's. i averaged 20.3 MPG driving 67mph (just under 2000 rpm on the tach), and used grey bottle diesel kleen. wife and i drove back out to KS the week after our wedding in june of this year. same weight cos we had about as much crap in the cab as i had on the way up there. same driving style. 67, using diesel kleen. only difference was it had the general grabber 265's on it. average economy was 19.7.

i have noticed a bit more of a difference in shorter trips and/or city driving. i plan on going back to the narrower tire size for my next set and after this last change just to a wider size, there's no way i'll go to a 33" tire, which i thought about for a bit.

the other thing to consider is what kind of TREAD you have/want. i went from a more highway tread to a more aggressive tread. if i can get general to pro rate a new set (they're wearing FAST), i'll try the same grabbers in 235/85 to see what they do. or i might try the transforce in 265. i had good luck with them the first round, and can be bought locally.