i own a 2004 chevy Z71 w/ 285 75 17 BFG TA KO tires. Everything else is stock. I have had the tires about a year and a half. About 6 months ago my gas mileage went from 17 mpg to 13 real quick. I have checked and changed everything simple such as fuel, oil, plugs and wires, air filter. My tires are running 50 psi. I take off slow and drive about 65 on the highway. is it my or my truck? and helpppp.