From: http://www.mp3car.com/vbulletin/engine-management-obd-ii-engine-diagnostics-etc/75138-calculating-mpg-vss-maf-obd2.html



For the record, the first "one-line" MPG formula above, taken from
my Circuit Cellar article, is off by 100!  The "4.54" should in
fact be "454".  The correct formula is:

MPG = (14.7 * 6.17 * 454 * VSS * 0.621371) / (3600 * MAF / 100) MPG
= 710.7 * VSS / MAF

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:

IMAP = RPM * MAP / IAT MAF = (IMAP/120)*(VE/100)*(ED)*(MM)/(R)

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.

