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Lightner's article, posted above by Magister, is very well written and in depth. He offers schematics to properly handle the vehicle's 12V power supply and an in depth examination of the J1850 VPW protocol. The end of his article claimed circuit cellar (magazine running the story) would cover all the protocols in the future, but I haven't been able to find them.
In any case, I found the calculations necessary to convert OBD-II PID's to MPG. The information is also posted in the main instrumentation forum. All from Bruce Lightner's mind:
The calculation depends on whether your car has a MAF sensor or not.
MAF Sensor:
MPG = (14.7 * 6.17 * 454 * VSS * 0.621371) / (3600 * MAF / 100)
= 710.7 * VSS / MAF
14.7 - grams of air to 1 gram of gasoline - ideal air/fuel ratio
6.17 - pounds per gallon - density of gasoline
4.54 - grams per pound - conversion
VSS - vehicle speed in kilometers per hour
0.621371 - miles per hour/kilometers per hour - conversion
3600 - seconds per hour - conversion
MAF - mass air flow rate in 100 grams per second
100 - to correct MAF to give grams per second
No MAF (Uses MAP and Intake Temp to approximate MAF):
IMAP = RPM * MAP / IAT
MAF = (IMAP/120)*(VE/100)*(ED)*(MM)/(R)
MAP - Manifold Absolute Pressure in kPa
IAT - Intake Air Temperature in Kelvin
R - Specific Gas Constant (8.314 JM/K)
MM - Average molecular mass of air (28.97 g/M)
VE - volumetric efficiency measured in percent
ED - Engine Displacement in liters
This method requires tweaking of the VE for accuracy.
I have no idea if these are the methods the SGII uses. The accuracy will be off due to approximations (AFR ~14.7, etc). All the information is from the brilliant mind of Bruce Lightner.
- LostCause
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