My gsr must be really messed up worse than i thought...I've recently replaced the air filter, plugs, wires, fuel filter, 02 sensor, run fuel injector cleaner, on-time oil changes and i check my tire pressures all the time and i am still only lucky to squeeze out about 20-21 mpg. This is even the case when using cruise a lot and trying to drive easy. My car currently has 134,000 miles. I know my thermostat needs to be replaced (its stuck open) and I know this can affect gas mileage but could it drop it this significantly? I have an aftermarket Skunk2 P28 ECU so I think I'm dumping a bit excessive fuel as a result. Plus, I think the previous owner disconnected and messed with a lot of the sensors on the car....