Reading through it the catch is without an MAF you need an estimate of volumetric efficiency (VE). I read elsewhere that for a modern 4-cylinder most people use 80%. I suppose you could fiddle with that number a bit.
In the thread, Bruce Lightner had the clearest explanation and here are his formulas:
Note that OBD-II VSS reading is in kilometers/hour and MAF reading is grams/sec times 100.
This formula works very well in a modern automobile because the engine computer spends almost 100% of its time managing the fuel-air-ratio to 14.7, which it can do very well because of the "closed loop" feedback from the O2 sensor(s).
In fact, the accuracy of this method has been proven in literally tens of thousands of gasoline-powered vehicles. Accuracy within a few percent is typical, often limited by the accuracy of the vehicle speed reading (i.e., VSS).
As for other ways of doing this, especially if you don't have a MAF sensor, by knowing the displacement of the engine, and after a simple "calibration" using fuel tank "fill-up" data to find the only unknown, namely the "volumetric efficiency" (VE) of the engine, MAF can be calculated from RPM, MAP and IAT. With VE, one can use the following formulas to calculate a synthetic "mass air-flow" (MAF) in grams per second, all without a MAF sensor, using the "Ideal Gas Law", as follows:
where manifold absolute pressure (MAP) is in kPa, intake air temp (IAT) is in degrees Kelvin, R is 8.314 J/°K/mole and the average molecular mass of air (MM) is 28.97 g/mole. Note that, in the above formula, the volumetric efficiency of the (4-cycle!) engine is measured in percent and the engine displacement (ED) is in liters.
The VE of my 1999 7.4L Chevy Suburban is about 65%. Smaller, higher performance engines can have VE's of 85% or higher.
The lack of injector pulse input from the OBDII explains why the MPG ends up as an approximation, because it has to assume a 14.7 to 1 A/F ratio as a basis for the calculation. This means it is true that the scangage would miscalculate the MPG if the intake air temp sensor was faked until the fuel trims reflected it.