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View Full Version : E34 MPG Computer Accuracy?


Gatsby1925
10-11-2009, 01:38 PM
Hi, all. I'm interested to know how the onboard MPG computer works in the E34 cars. I mean does it measure actual gas consumption, or does it work by estimating mpg via car speed, rpm, distance and the like? If it measures actual fuel consumption over a distance that would be much more accurate a means. Thanks!

woodnthings8
10-11-2009, 06:11 PM
Mine is very accurate. When I fill up I reset my trip odometer, I also reset the mpg calculator. When I drive 350-400 miles I usually fill up again. If I manually calculate the mpg I usually get withen a 1/2 mpg of what the computer shows on the dash. The mpg indicator in the gauges seems to use fuel flow vs. mph to give the indication which is why it is so accurate.

Gatsby1925
10-12-2009, 03:06 PM
Mine is very accurate. When I fill up I reset my trip odometer, I also reset the mpg calculator. When I drive 350-400 miles I usually fill up again. If I manually calculate the mpg I usually get withen a 1/2 mpg of what the computer shows on the dash. The mpg indicator in the gauges seems to use fuel flow vs. mph to give the indication which is why it is so accurate.


Thanks for your usual helpful input, Woodnthings. Much appreciated.

attack eagle
11-01-2009, 07:59 PM
Hi, all. I'm interested to know how the onboard MPG computer works in the E34 cars. I mean does it measure actual gas consumption, or does it work by estimating mpg via car speed, rpm, distance and the like? If it measures actual fuel consumption over a distance that would be much more accurate a means. Thanks!

it measures the actual fuel used vs speed. You can watch your fuel usage in real time in liters per hour on the OBC. For example idling in my m50NV was 1.1 liters per hour, but idling with the a/c on was 1.7.

if you know the fuel pressure, engine vacuum (FPR is vacuum referenced), and pulsewidth, it ain't that hard to calculate with considerable efficacy the amount of fuel being injected, and that is exactly what the OBD reads.