How to "calibrate" OBD-II derived MPG when using IMAP
"Plug-and-play" MPG calculation from OBD-II bus parameters is not out of the question for many vehicles---not just MAF-equipped ones. It all depends upon what is available on the OBD bus---from legally mandated OBD-II parameters (i.e., "emissions-related") and the much richer set of manufacturer-specific OBD parameters. Here are some things to consider if a MPG device wanted to make "guesses" about your non-MAF vehicle's engine and gas tank and then "plug-and-play" using IMAP data...without initial user input:
Originally Posted by Aberk
(1) Read the VIN (now a legally mandated OBD-II parameter). With "VIN decoder" firmware (a non-trivial, moving target in itself) one technically can find engine type and displacement, and even fuel tank capacity---as well as paint color!? :-)
(2) Monitor the fuel tank level (now a legally mandated OBD-II parameter). For many vehicles, old and new, this parameter can be read as a manufacturer-specific OBD PID. Knowing (or guessing) the fuel tank capacity, over time (watching for "fill-ups") this can give you what you need to calculate MPG---despite the well known non-linearity of everyone's fuel gauge!
(3) Count the number of O2 sensors and make a guess about the engine displacement and/or fuel tank capacity (e.g., V8 engines have more O2 sensors).
I have some more, but this kind of stuff is potentially valuable, as more and more States---and even the Federal Gov'ment---are looking at a direct "road tax" on our driving. Why? Because per-gallon fuel tax receipts are dropping, as more high-MPG vehicles (e.g., hybrids) hit the road. For example, our friendly government "overlords" need some way to calculate mile-by-mile fuel consumption so that the per-gallon fuel taxes somehow can be refunded to "Big Brother Box" equipped vehicle owners! :-)