I will definitely stop 'topping it off' then, at first I thought it might be a venting issue with the car - Back pressure shutting the fuel pump off too early, but when I looked up how the gas pump works I decided otherwise. I pasted it at the end if you get curious. I liked this explanation best because it used Bernoulli.
Well I can say i feel lucky that my gauge works then because I pushhed it right after I got it and ran that car out of gas. I will start a new MPG thread - Like I mentioned I am getting a solid 20 and that is with a heavy foot on the weekends (I drive lighter during the commutes to work).
Randy
In a gas pump handle you have two valves: the main valve, which is actuated by the oversize trigger you squeeze to make the gas flow, and the check valve, which lets gas flow out but won't let anything back in again, thus reducing fire hazard. In the seat of the check valve you have a little hole. To the backside of this hole is connected a Y-shaped tube. One branch of this tube runs down the nozzle and exits at the tip while the other runs back to a diaphragm connected to a release mechanism on the main valve. When you squeeze the gas pump trigger, gas running past the hole in the check valve sucks air out of the Y-shaped tube. (This is because of the Bernoulli principle: a moving stream of fluid tends to pull things in from the sides. Take my word for it.) As long the end of the Y-shaped tube exiting at the spout is unobstructed, air is simply pulled into the tube and nothing much else happens. However, as soon as the gas in your car's fill-up pipe gets high enough to cover the end of the tube, a partial vacuum is created therein, which yanks on the diaphragm, releases the main valve, and shuts off the gas.
