dcb,
your fuel pressure is what 0.5 psi maybe, first i thought, graduated cylinder with a stop cock on the bottom. Then i thought an IV drip in the fuel line counted by an electric eye. and magnet on a spoke counted by a sensor.
we are going to need an electronics wiz to jump in here sooner or later and tell us how to divide one signal by another and display the result. that does not give us mpg but it would be a relative number. for comparing driving styles or whatever.
how about a solenoid style fuel pump, they pulse on and off
http://www.facet-purolator.com/solidstate.asp