I'm thinking about putting a set of 315 Terra Grapplers on my '99 F250 PSD. I'm curious as to how much fuel mileage I would lose over the 285s I have on it now. Anyone gone from 285s to 315s and checked the mileage?

What gears do you have? When I went from 265's to 315's with 4.10's I went up a couple MPG's.

The axles are 3.73, and it has an auto tranny.

the superdutys are geared really low to begin with, especialy the F350. going bigger in tire did not affect my mileage at all, and i went to 305's. but for towing's sake i went back down to stock size. it's easier to get D/E rated tires at a resonable price in the stock size.