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MPG can be calculated using speedometer and fuel used info. It is more reliable to use injector "on" timing than the amount of fuel remaining in the gas tank. That's because the tank float mechanism generates a varying signal for fuel level when the vehicle moves and fuel sloshes around inside the tank.
Ford EEC-IV and EEC-V computers have a fuel used signal that tells the trip computer how much gas has been burned. (Years ago, I spoke with a Ford service center about the wiring for the digital instrument cluster. They told me the fuel used input was pulled HIGH to 12V at up to 500 Hz. This signal corresponded to 48000 pulses per US gallon of fuel used. This jives with a patent filing I also found for measuring fuel used.)
If you don't have something like that, you might be able to get injector pulse width information from the PCM. If you know the rate of fuel flow in your injectors, you can calculate fuel used. Injectors are rated in pounds per hour. The injector is turned on for a number of milliseconds. The "on" time determines how much fuel flows through each injector. If I'm not mistaken, all injectors should have the same "on" time for a given throttle setting.
Once you know the fuel used, get mileage from your speedometer input. Nearly all cars have a speed sensor that generates so many pulses per mile travelled. Just count the pulses and divide per time (i.e. pulses per second). The MPG then equals the number of miles travelled divided by total fuel used.
Instantaneous economy is the MPG value updated each second. Average MPG is the instantaneous value averaged over a period of time. This time is the elapsed time since the Avg MPG value was last reset.
Hope that helps!
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