The fuel quantity is usually determined with the tanks full to the top, just over brimming. Unusable fuel is determined by flying until the engine quits. then draining the remainder. Unusable fuel in the stuff below zero on the gage, but is included in the empty weight of the aircraft as "trapped fluids"
Regarding fuel consumption rates, use the flight manual, and check by topping off at the beginning and end of flights, recording the fuel bowser meter reading (they are quite accurate!)
Topendtorque's mysterious lesser fuel burn on hot days is easy enough to explain - on hot days the engine produces less power, and the engine burns fuel to make power. Less power, less fuel burn rate!
Last edited by ramen noodles; 14th Sep 2008 at 15:20.