Hi Y'all,
Even non computer controlled carburetor cars can give accurate instant MPG readings. You will need a pulse-generating flow meter and a counter. Activate the counter when you pass a 1/10 mile marker and deactivate it when you pass the next on your test loop. Now get out your calculator and figure the cc of fuel consumed by pulses. Now you have the raw data that you need for mpg: fuel consumed and distance traveled (BTW: fuel temperature will affect fuel density).
If there is a tank return fuel line you need two meters and if you want instantaneous mpg you need a pulse for distance from the VSS or a hall-effect ring on the driveshaft. Then a programmable processor with three high speed counters, fuel temp probe and a display interface. Badda Bing $$ there goes your budget.
It just depends on how badly you want this information and what you will pay to achieve it.
To be honest, the best measure of average mpg per tankful is my wallet.
Cowspots