I think the larger tire gets slightly better highway mileage because it rolls farther per revolution, but this is directly offset by the extra air drag of being higher up into the flow with more of the undercarriage exposed.
In other words, I think I get exactly the same mileage with 31's as I now get with 33's taking the difference in odometer readings in account.
I have a regular 200 mile run that I get to check gas brands and octanes out on. Different brands of 'fuel' make up to 10 mpg difference in my CJ7!
My speedometer and odometer are exact with 33's on her according to many GPS checks.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT'sKen Knapp wrote: