plenty of things can contribute to bad gas mileage but yeah if you haven't replaced the fuel filter in awhile you should. my last replacement brought about a 3mpg increase in both city & highway. what's wrong with your thermostat is it sticking? i had the mechanic replace it so i can't tell you how to do it but good luck as that is a repeated offender on this car.
well every few times i cold start my car after like 5 minutes of driving it the engine temp. goes to its overheating but it cant be. so i asked around and people said it was the thermostat