Correcting OBC fuel consumption through unlocked OBC....

[Scott H]

Well, after a couple thousand miles on the 535iT, I am seeing 19mpg consistently (80% highway, 20%city). My OBC shows me 14.7 to 15.3 depending on where I am at in the running average. This is roughly a 24%-28% error. A Chicago area independent swapped an M30 + auto in to a 1992 525iT at the same time I did my project. He estimated 24% error on his about a month ago......so we seem to be on the same page. I think the reason for his finding being lower is that he does a lot more city driving than I do. The fact that my engine is pretty much rebuilt while his is a 135k mile M30 might contribute as well.

According to OBC function 20, I should be able to put a correction factor in to make the OBC display correspond closer to my actual fuel consumption......see link.

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

In my 11/88 build 535 I could never get this function to work....the value remained 1000 no matter what I tried. I'm attributing this to early software of the 535 and assume my 3/92 should be able to work.

Anyone done this successfully, or tried and failed?

According to the instructions, I should find success by either calculating:
19/15.3 * 1000 = 1,242
or the reverse
15.3/19 * 1000 = 805

The first option seems more likely the way my brain works......but then again my brain has definitely been known to stop working.....often.

Thanks!
Scott