my BOSCH MAF gives a voltage signal 0-5v. this is calibrated to air flow rate which is directly proportional to fuel flow rate.

boscheSensors (page 57)

all we need do is data log voltage and speed and we have a method of calculating MPG. or alternatively you could just display the voltage on a meter and do the mpg calculations in your head! at constant speed you know what voltage to expect, if mods reduce the voltage then you know they save fuel.