Im driving a 09 chevy 3/4 ton 4x4 6.0 crew cab with 285/75/17 tires axel 3.73 and tranny is a 6speed HD automatic. I try to run 91 - 92 grade fuel but since prices r so high I run 87 grade. Right now I get 13.4 mpg I drove for about six hrs on the interstate n it went to 13.6 whn I test drove this truck it was.saying 14.7 mpg. Why did.this.drop?? What can I do. Can somebody recomond a.programer mass ari.sensor n air intake... mabe I can do something with my.exaust? ? Please HELP!! THX