MPG gauge-How does it work?

[Russell]

Or more accurately, what does it measure? is there a good source for this info?

Mine seem fairly accurate when compared to traditional tank to tank filling.

Asked the archives in a general way and nothing of importance came up. TIA

[525i!]

someone suggested to me that it was a simple vacuum guage. but I think that it must read more than that judging by the fact that if you are stopped it goes to zero

[Alexlind123]

someone suggested to me that it was a simple vacuum guage. but I think that it must read more than that judging by the fact that if you are stopped it goes to zero

Vacuum guage could measure straight fuel consumption. Perhaps it just uses that number along with vehicle speed?

[Jeff N.]

The gauge is driven by the fuel map in the ECU. Essentially, the combination of the engine speed, air velocity (AFM) and a few miscellanous other things (engine temp, air temp, blah blah) locate a fuel value in the chip.

This value equals a calculated fuel pulse. The fuel pulse and the road speed combine to give you a calculated fuel MPG calculation.

Simple huh?

[Russell]

Thanks, this is detailed enough. There was post on BOBISTHEOILGUY forum about "MPG gauges" and their inaccuracy. I have always noticed mine seem fairly accurate. I had believed it was based on more than a "vacumn gauge", not I have a better idea.

[calmloki]

Thanks, this is detailed enough. There was post on BOBISTHEOILGUY forum about "MPG gauges" and their inaccuracy. I have always noticed mine seem fairly accurate. I had believed it was based on more than a "vacumn gauge", not I have a better idea.

If you find that the digital gauge in the computer disagrees with computed mileage (drove x miles, used y gallons, mpg = x/y) you can enter a fudge factor to the computer - I've been playing with that in an attempt to get computed mpg = computer's mpg. My ASSumption has been that the analog gauge is driven by the computer and that the fudge factor would be applied to it as well. The differences I'm trying to erase are small enough that they wouldn't be seen on the analog gauge though.
Tom

[Morgenster]

I'm pretty sure it calculates this with fuel delivery values in the ECU because mine operates just the same wether I drive on LPG or petrol.

When driving with LPG, fuel delivery is cut off, so if it would take info from a sensor it would drop to zero and stay there, which it doesn't.