Originally Posted by
JojoJaro
On Diesels, the fuel pump (AKA Lift Pump) delivers the maximum fuel rate the engine could possibly need all the time. The IP (sometimes AKA High Pressure Pump) regulates the fuel flow to the injectors and returns the excess. The injectors themselves also return the extra fuel it does not need. (I suspect this is true for fuel-injected gas engines also. For carb engines, I don't know.)
To accurately measure fuel consumption in all conditions, you need to measure the feed flow and subtract the return flow. If your flow meters are accurate, you should get a steady-state fuel rate consumption. Instantaneous fuel rate consumption might still be a little tricky since it takes a little lag for the return flow to keep up with fuel consumption.
On the other hand, vehicles with OBDII already has many of this information. You can retrieve this info and calculate you instataneous MPG. This is how a ScanGauge works.