You are going to pay a bit for the gears. I am looking to put in 4.56 because I am running 37" and I have found a couple places that will do it for $1200. But putting 4.11 for 35" will help out some. How much is hard to say, but you should get back at least 2 mpg. And 14-16mpg is better the 10-12mpg.