Thread: Calculate MPG for diesel with OBD2 PIDs, how?

Originally Posted by Magister

I know the formula for gasoline engine, mostly using MAF+VSS.
For diesel, MAF can not help, but using MAP, IAT, RPM and LOAD we can evaluate the MPG.

Anyone know the formula? I understand that LOAD for a diesel engine is based on current fuelflow / peak fuelflow @ sea level, but I miss something somewhere :-/

Magister,

Did you ever find an answer to this? I'd like to do the same calculation. The post here suggests that you can assume an Air Fuel Ratio of 14.5 and use Lambda, or the equivalence ratio (pid 44), to correct it. However, this would use the same formula as for a gasoline engine, which sounds like it may be incorrect.

If you ever figured it out, I'd greatly appreciate the help.

I given my arguments against MAF etc before (use injector pulse width instead), but as I understand diesels, using MAP etc would be even worse since diesel lambdas often vary from 3 to >20 under normal running.

I also find some cars on the market don't support the MAF (read by OBD scanner,return NO DATA).Is it possible calculate the fuel cost by MAP on the gasoline cars which don't support the MAF ?

I discussed this whole mpg thing with one of my gurus that I hadn't seen for over 3 years.

He reckoned of course injector info can be read and then showed me the relevant PIDs.
Sure enough, there were IMO likely candidates under the "scaling" heading, though I did have some concerns. (I have little knowledge on buss info - I can't find the OBD connector on my 1965 ute anywhere! )
FYI - he had a Mitsubishi.

He also said that MAFs were superior to MAPs, but again, IMO both give a reasonably non accurate economy reading.