Abstract: A method for determining an approximate, instantaneous fuel economy of a gasoline powered motor vehicle. The method involves determining a nominal fuel flow through fuel injectors of the vehicle's engine under static pressure conditions and adding to this value a determined additional quantity of fuel flow through the injectors which is caused by dynamic conditions such as engine rpm, vehicle speed, etc. The additional quantity of fuel flow is determined in part by monitoring a pressure drop across the intake manifold of the vehicle. The temperature of the fuel and the octane rating of the fuel are estimated and used to help determine a fuel density value. The fuel density value is then used to determine a total volume of fuel used. The total volume of fuel used is then ratioed with distance information to provide a fuel economy value.