Oh! I missed the flow sensor. I had been discussing FE calculation on various types of car with an electronics hacker friend of mine, and I guess I got pulled into that in my head.
With a flow meter like that on your fuel line, you might be able to work something out. If your car has a fuel delivery system with a return, you will need two of those sensors--the pump will keep pushing fuel at all times, and anything not needed by the engine gets dumped back into the tank. So you work on the difference between the accumulation of "ticks" from the two meters.
Your fuel flow signal will be the amount of fuel going into the float bowls of the carburettor, which is not necessarily the same as what is going into the engine. Though it should be a reasonable approximation, if a bit laggy.
With a speed readout from some kind of sensor on a brake rotor, or on a CV joint, you'll be able to get a distance reading, and be able to calculate MPG (or l/km) from that. Should even be a fairly simple calculation, which is nice.
You can obviously use an Arduino and the LCD with buttons to do this. Wiring it and programming it would be a little more tricky, but I bet you could get at least pretty close with what is used in the MPGuino.
Cool, I think it will work! But I don't know exactly how...
-soD