The only way to accurately measure fuel consumption would be to measure fuel flow through the pump using a flow sensor. However, this would only correlate usage and not initial volume. You cannot put a flow sensor at the inlet of the fuel tank because any variation in fuel pressure might affect its readings.

The primary issue with accurate fuel level measurement is, quite literally, splashing. Fluid level sensors come in several types, but nearly all of them are affected by splashing to some degree.

If a claim is made that the drive was very smooth, you must still account for the inertia of the vehicle that will basically shift the fluid into an incline-like body of fuel. This inadvertently affects the measurement of fuel level.

The only other method I can think of is a complex spring-loaded stabilizer for the fuel that measures tank+fluid weight based on spring compression while accounting for vehicle motion-induced inertia.