I have a 2002 GMC Envoy SLT. The fuel mileage is absolutely horrible. I get about 12.5 miles to the gallon and I owned an 05 ext trailblazer before that got 16-18mpg. I have replaced the plugs, cleaned the intake and throttle body, changed the air filter, replaced both the upstream and downstream oxygen sensors, replaced the fuel filter, changed the oil, replaced the thermostat because it was stuck open, and replaced a few of the coil packs because they needed replaced. I have used sea foam in the gas tank as well as in the top end. I still get 12.5mpg!! I know this can’t be normal, what else can it be? I do on occasion get a P0440 Evaporative System leak however it runs fine. Gas cap has also been replaced. What else could that be and could that be the issue as far as fuel mileage as well? Unrelated I replaced the rear sway bar links because they were broke, as well as the rear lower trailing arms because they were broke as well. The shocks are relatively new in the rear as well as the springs. Tires are inflated properly. 35psi all the way around. Rated for 32 psi on the door panel but I like to run at least 35 psi. The brakes were also recently done all the way around including rotors and the left rear caliper was replaced as well. What am I missing? I should get at least 16mpg I am sure of that!