Q:
A: Very little I'd say. In regards to the above answer that you would get more difference on highway mileage my experience differs. I can still get 24mpg (UK) with 33" Goodyear MTRs. The best I got with my Jeep before any modifications was 24 mpg. It's city driving, with stop starts, where the bigger difference is felt It takes a lot of energy to increase the rotational speed of tyres than the stock size. Stock I averaged 18.5 mpg now I get 16.5-17.5. So 1-2 mpg difference in city driving. No difference for highway driving for me. And remember that's comparing stock tyres to 33" MTRs. The difference between AT/MT tyres of the same size will be very little. In fact the thing that causes most difference in MPG for any vehicle is driving habits.