I don't know about that...the onboard MPG in my Cad runs fairly close (not exact, but close).
It'd be interesting to see a comparison between the onboard MPG and the Scangauge......
my reply would be, why would Infiniti, in their flagship sedan, send less reliable information to the onboard MPG, than the information being sent to the OBDII port? seems silly....
I have installed aftermarket mpg systems (20 yrs ago) that were not accurate but I really think the info being fed is consistant and accurate.
with that said, I AM running 255 and not 245/45/18 tires so I know that it is off. But I'm ok with that beacuse I did not bother to have the system recalabrated.
I am looking forward to all the other functions though!
There's no motive for manufacturers, including Infiniti, to make their MPG display particularly accurate. However, there's no reason for them not to, either. I'd guess that its accuracy depends mainly on their engineers and how they put it together.
There is no MPG information that goes to the OBDII port. There's not even fuel rate. Speed, at least, goes to the OBDII port. Things like the ScanGauge have to guess by calculating from other known factors what the fuel injection system is likely to do. It probably uses MAF, O2 and IAT to calculate its fuel rate guess.
Therefore, a Scanguage is likely to never match an OEM MPG gauge exactly, since the OEM unit probably has access to fuel rate data.
On my Cad, one of the things you can see on the computer is the injector pulse length, in, IIRC, milliseconds. This may be what it uses to calculate fuel flow rate. Does OBD II provide, and does Scangauge show similar data?
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No. OBDII does not provide injector pulse length, duty cycle, or any other way to directly measure even relative fuel rate. There may be manufacturer-specific extensions that do but the ScanGauge (and other OBDII equipment without manufacturer-specific extensions) cannot access them.