Determining Miles per Gallon

**PredatorCoder** · 09-13-2005, 01:22 PM

Based on the inputs from the OBD-II sensors, how would you determine the current (not average) miles per gallon you are achieving? All I'm interested here is a formula (or more than one formula if multiple ones exist). The programming part I can handle.

**Curiosity** · 09-14-2005, 06:16 PM

StevieG did this a while back, but I don't know what to search for. Basically, it's speed / MAF * fueltrim. I can find the thread if you need it.

**PredatorCoder** · 09-14-2005, 06:33 PM

Is it possible to pull this off if the ELM-323 OBD-II adapter I'm using doesn't report a number for MAF? I get the fuel trim, vehicle speed, and RPMs but the MAF sensor reports "N/A".

**Curiosity** · 09-14-2005, 07:17 PM

Oh, that's not good. You'll need the mass air flow to figure air / fuel ratio. The MAF is Mode 1, PID 16 (10 hex). You sure it's N/A?

**PredatorCoder** · 09-14-2005, 07:44 PM

Yeah, I think so. I'm using ScanTool 1.08 from http://www.scantool.net/software/sca...et/index.shtml.

Several other sensor readings come in on the first screen, just not that one.

I do get these values from the first page:

Absolute Throttle Position
Engine RPM
Vehicle Speed
Intake Manifold Pressure
Fuel System 1 Status
Fuel System 2 Status reports as unused all the time.
MAF sensor reports as "N/A".
I think I also got calculated load value & timing advance.

On the second page:

Short and Long Term Fuel Trim (Bank 1)
Bank 2 fuel trim readings come in as "N/A".
I think I got the intake/coolant temperatures as well.

**Curiosity** · 09-14-2005, 08:06 PM

It looks like you have a 4 cylinder. I would think MAF would always be supported though. Is there any way to find out what Mode 1, PID 0 is? That's the PID support bitmask.