I have 295/70/17D tires and am considering 255/80/17E tires. Both are aggressive "mud terrain" type tires. Do you think the difference in width will make much or any difference in mileage? They are both basically 33ish in height.

 

Maybe.

Your new tire will be narrower, but may be heavier. Once cancels the other it would seem. Looking at specs on Tire Rack, it looks like they may be the same weight.

 

I wonder though if they will have a different amount of rolling resistance and therefore have a different impact on mileage. But, I dunno.

 

The load range E will have less rolling resistance when fully inflated due to having a stiffer carcass. Being narrower also, you'll probably see another 1-2mpg on the highway, and no change around town.