[Christ]

I'd think, given the nature of the competition, that they don't "short fill" the tanks.

I'm sure they rather fill them up to begin with, let them go either until they run out, or until they reach a pre-determined distance, and then measure either the distance where they ran out, or the amount left when they reached the end of the "track".

In this way, you eliminate the "short fill" aspect altogether. If you're starting each contestant with, say, 2oz of fuel, and you can accurately measure a weight change down to 1/100 of an OZ, you can probably tare weigh the "tank" (probably, all of which are uniform within x-factor anyway) and then just weigh the amount of fuel after filling and after running. The difference would give the figure for the usage amount, and could be converted into usage/distance figures.

Rest assured, those 2-3000 MPG cars, they'd stall 50 feet from a gas station from empty tanks, but start running again as soon as you got close to the pump... from the fumes wafting in the air.