Hard to say on MPG. I installed it the same weekend as the RR lift and 265's from stock. Of course the stage 1 requires 89 octane so that mixes things up too. I can tell you that going with larger tires I did not see a decrease in MPG. Now that I have the 4" w 285's I am seeing a decrease in MPG - but that's expected. With the RR lift / 265's I could average mid 17's on the highway and around 14 in the city - not bad at all. Now I am getting 16's HWY and 12 city with the 4"/285's