I've got 3.55 gearing in my truck. If you think that going from a 3.55 to a 3.31 will yield you huge gains in MPG then you are mistaken. I'd like to know what gearing the truck had that got the 32MPG that they advertise on every Ecoboost commercial ever made?? Every one I talk to has said their "dealer" told them the same thing about the mileage getting better but have you actually met a real life person that has gained any more than 2-3 MPG after break in? I've heard the rumors and know a guy who's sister's cousin's dog groomer's neighbor had an Eco that got an extra 5 MPG after break in, but I'm not drinking that Koolaid. I simply made the comment because no-one has any actual proof to back up these claims that the dealer makes and I wanted the OP to know what he was getting into. I went through the same thing with my 6.4 F350, dealer swore up down and sideways that mileage would get better after "break-in". The only thing that increased MPG on that truck was a new exhaust and a tuner.