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If you want to go the cheap route, forget about all the flow measuring junk.
Get a decent sized 16v capacitor, maybe 100,000uf.
Take a line from the fuel pump's + terminal.
Run it through a variable current regulator, through a diode to the + of the capacitor. This is called an integrator.
Use a very high resistor (1M+) between the capacitor and a DMM to measure your fuel usage at the end of your trip. Do some trial and error and find the right current value so that 1v corresponds to 1gal. That way, the meter will be good up to 12 gallons. For a reset button, all you need is a button wired through a 10 ohm resistor between the terminals of the capacitor.
Keep a little four function calculator to get MPG numbers at the end of your trip.
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