you didn't say if you were 2 wheel drive or 4x4, but to give you an idea here is the 235-75-15's on a stock '04 2.3 l 5 speed 2 wheel drive and i average 23-25 mpg my door jam sticker says i should have 225-75-15 for comparison with that size tire i was getting 26-27 mpg. here's a pic.