Don't go chasing bad MPG unless you are sure of it. You could be spending time and money chasing ghosts. So try this experiment firsta) fill the tank to the max(b) get on the HWY - late night / no traffic hour(c) drive about 20 to 30 miles (at 55 MPH)(d) use cruise control - very important(e) note the EXACT MILES(f) get off and immediately get gas(g) note the EXACT GALLONSNow you can calculate the MPG byMPG = MILES (step e) / GALLONS (step g).The reason you start out with full tank is that is the "repeatable" condition. You want to avoid traffic so you don't use the brakes on the HWY. Braking will negatively affect the MPG calculation. You use the cruise control so you and eliminate your driving habit as a factor.Without a good MPG calculation you might be waiting your money chasing ghosts. Also you can repeat this test anytime you feel like you car is not running right. You can also try the same thing but off the cruise control to check how much your driving habit affect the MPG. etc.