Average MPG on trip computer

[repenttokyo]

it's an average of your last x hundred miles of driving.

[ahlem]

If I rely on the trip computer I get 15.7 MPG. If I do the real math I get 22.1 MPG. I do have a 3.9x diff. Is there a calibration issue?

[Russell]

mine was accurate with in 1/2 mpg the last time I checked it with actual consumption.

[Russell]

If you mean it resets every xxx miles, I think you are wrong. It is only reset when you reset with the S/R button.

[repenttokyo]

If you mean it resets every xxx miles, I think you are wrong. It is only reset when you reset with the S/R button.


that's not what I mean. What I mean is that your car looks at the last X hundred miles of driving and presents the average over that time. If you hit the reset button, it starts it's count for the average period again. I seriously doubt that the car remembers the entire fuel consumption and mileage between resets, it is most likely only looking at a specific time period within the reset window. that is how these systems usually work.

[BMWCCA1]

No reason for it to be off unless some messed with it, or replaced it, but you can recalibrate it. Search this site for "obc hidden features" or take a look here: http://home.iae.nl/users/bts/obc.htm

[Black 535i]

...I believe that the onboard computer figures out averages and you are figuring out the 'actual' average as I do. I too am way off the onboard computer averages everytime. It takes into account how you drive, how you accellerate and all that whereas we just figure out the average of mileage divided by gallons used and not how they are used. Anyone else have a theory on this?

[billy]

if you select mpg on the OBC, you should notice that the average will reset every time you press s/r button, therefore you would just start getting the mpg average from now onwards rather as say the last 100 miles

[uscharalph]

that's not what I mean. What I mean is that your car looks at the last X hundred miles of driving and presents the average over that time. If you hit the reset button, it starts it's count for the average period again. I seriously doubt that the car remembers the entire fuel consumption and mileage between resets, it is most likely only looking at a specific time period within the reset window. that is how these systems usually work.
It does calculate based on miles driven since reset.

[uscharalph]

The one calculation the obc does make based on a time frame is RANGE. It calculates fuel on hand by mpg over the last 20 miles driven.

[RobPatt]

....'94 525ia with 152K miles.... no a/c on... windows up... only fresh air coming in via the vent.... very happy with that mileage...as was measured by the computer.....

HOWEVER...as my tank went 1/4 full and below... the range function seemed to get more and more wild... which makes sense in a way as there's less fuel to calculate from so the "driving style of the moment" has a greater influence on actual range possible....

anyway 26.x mpg I'm happy with... Went round trip from Biloxi, MS to St. Pete, FL.

[Dave M]

It does calculate based on miles driven since reset.

This is how I understand it to work as well. If one doesn't reset the CON2 for example, it will take significant time to affect the average. Following an 8000km trip, it took over amonth of driving to bump the CON from 7.9 to 8.0 L/100km.

Dave M

[BradHouser]

I seriously doubt that the car remembers the entire fuel consumption and mileage between resets, it is most likely only looking at a specific time period within the reset window. that is how these systems usually work.

It would be way more difficult for the OBC to remember how many miles and how many gallons over a sliding range than to remember total miles and total gallons since reset. You would need many more memory registers, as opposed to only two.

With this simplifying assumption you get:


Average MPG = Total Miles/Total Gallons since reset.

I seriously doubt it looks back a fixed number of miles. That is why there are two, one you never reset, the other you reset for a trip.