[MetroMPG]

Potential problems to evaporative emissions systems was mentioned as the reason for going with the 1-click fill method.

(My personal method has always been "fill to the brim". My car has evap, and it's had no problems, according to the ECU.)

Another option would be to just weigh the vehicles before/after. You'd need a pretty accurate scale though. (And would have to trust the competitors not to scoop up a handful of gravel or something, part way through!)