I forgot to mention that in that scenario, the attendant wouldn't even have to override the system. If the pump detects and can converse with the GPS, it charges based on the mileage. If not, the huge per gallon tax goes into effect (stick it to the out-of-staters, who have no choice, dodge a couple political pitfalls, and discourage people from disabling their GPS's). Nifty neato, huh?
That may or may not be Constitutional. Taxing in-state and out-of-state people at different rates has always been a Constitutional hot-spot.