OBC AVG MPG is too high

[danv]

My car died on the highway about two weeks ago, I had it towed to a reputable bmw shop and they replaced the ignition coil, it runs great again.

My problem: the OBC AVG MPG reading is way too high now. I used to average 21-22 mpg, and it matched my own calculation (trip odometer / gas pumped in.) When I picked up the car it was a little high, I thought from the tow, but it has kept going up. It was up to 27mpg at the beginning of my morning commute, so I decided to reset it. That shop must have done the hidden prius drivetrain mod because the OBC says 37 mpg after 15 miles of mixed driving.

Before I try to figure it out myself and/or take it back to the shop, has anyone seen this before? Any quick fixes, places to check, etc? The car is a 1990 325is.

Thanks,

Dan

(Yes I searched).

[87bimmerguy]

Interesting. Mine has always read high but only 4-5 higher than my calculations. I have no idea why though.

[Kerrath]

Well, mine likes to read about 1.7x higher than I actually get, but it's an m50 swap and I doubt they could've pulled that one over when you weren't looking. :p

If you've only driven 15 miles so far on it, I recommend driving it a while before deciding it's a definite problem. Usually right after I get gas, I drive on the freeway for example for 20 miles, then even after like 40 more, it still will hover around 44mpg and by the end of the tank gets to 37.8 or so.

I don't think RPM is factored into the econometer reading, so I'm not sure how the coil replacement could affect anything.

[guibo09]

isnt there a calibration screw at the bottom corner on the face?

[danv]

In total I've driven about 140 miles, but you're right, I'll wait until I fill up again before I really do anything.

It's a stock car, and was reading right before the ignition cylinder, so I'd be surprised if it has to be recalibrated or anything

Well, mine likes to read about 1.7x higher than I actually get, but it's an m50 swap and I doubt they could've pulled that one over when you weren't looking. :p

If you've only driven 15 miles so far on it, I recommend driving it a while before deciding it's a definite problem. Usually right after I get gas, I drive on the freeway for example for 20 miles, then even after like 40 more, it still will hover around 44mpg and by the end of the tank gets to 37.8 or so.

I don't think RPM is factored into the econometer reading, so I'm not sure how the coil replacement could affect anything.