On average I get around 150-180 miles on a full tank, with an average of about 12mpg. Replaced my spark plugs last week and got 160 miles on the last tank. I drive almost daily in the city and I don't get the chance to floor it too often either. I turn the car on and off multiple times a day as well. Could it be that an O2 sensor is starting to go bad and the car is burning too rich? The exhaust tip is completely covered in soot. I have no lights on the dash and my transmission is just fine. Tried some fuel injector cleaner and didn't really make a big difference. Is there anything that could be affecting fuel economy?