265/75/16s are around 31.65" tall and the 255/75/17s on my 08 are around 31.75" (with current tread wear) and I get 14-15 MPG city and 15-17 MPG mixed city / highway driving.
Tread style will also make a difference. A/Ts will get slightly (.5 MPH) better mileage than M/Ts of the same size.
Here is an interesting read on this topic on JP Magazine: How Tire Choice Affects Gas Mileage - Jp Magazine
What size tires do you have now and what is your current MPG?