it depends on what gears you have I have 31x10.50's with the 3.92 rear gears and still get 20-22 on the HW it also is a 5 speed with the 4.7 but most likely you'll lose mileage with the bigger tires at least in the city.

 

i lost a lil in town when i went to 275 45 r 20 when my stock was 255 70 r 15

but my highway went up a lil...