OBC - Fuel Consumption

Can anybody please advise how my 1989 BMW 730i determines fuel consumed? The OBC calculates an average of 24.1 mpg on a good run, the instantaneous consumption needle sits between 20 and 50 mpg accordingly - but I'm actually consuming 10 miles per gallon of fuel purchased.

TIA

Yes, the OBC isn't the most accurate thing. It does have an adjustment of sorts, but how it calculates it's milage is the position of the throttle and how much fuel would have been consumed by that throttle position, and also factors in speed and RPM.

Depending on what version of OBC you have, you can make the adjustments accordingly.

http://home.iae.nl/users/bts/obc.htm

The one you're looking for is OBC Hidden Code 20.

Thank you for the explanation. I can't reconcile this though with what I see when I switch from LPG to Petrol, the needle instantly changes to indicate approximately half the previous MPG before speed/revs/throttle setting are changed (though the engine does start to misfire dreadfully on petrol).