I have a 7.4 Vortec w/ just a K&N air filter- throttle body spacer and flow master series 70 mufflers on it- what else can I do to up it's performance? It isnt a real go getting and MPG bites the big winny 9 MPG in town maybe 11-12 on the inerstate .. it has 4 10 gears as well its a 3/4 ton crew cab.. looking for suggestions