My gas light comes on 233 miles in city driving. I drive to about 279 miles then I fill up. I fill about 15 gallons and the tank is full. Which works out to be 18.6 MPG in the city. Is this correct? When the tank is full I get about 27 miles on the first quarter tank. Then it slows down a little. I find I use my trip meter as a guide and not my gas guage. Can it be calibrated? Any thoughts?