Speedometer running high

[pnosker]

I've GPS synced my speedometer and it turns out that the speedo reads exactly 2.8 mph fast at all speeds. Otherwise it's spot on. The OBC if I zero the "avg speed" mode gives me the right speed too. Anyone know how to fix this? I have no idea where the speedo and OBC get their information from. It's a constant though, and it's throwing off my gas mileage numbers I think. I always read high.

[1996 328ti]

They all do that. It's normal.

[pnosker]

Could I add a resistor to the sensor or something? I have no idea how the speed sending unit (in the diff, I think) works.

[John Firestone]

The speed sender signal is digital so tweeking its resistance wouldn't help. You could try a digital speedometer recalibrater like those from Dakato Digital, but that would only be exact at one speed, and would under- and
over-compensate at lesser and greater speeds. It would also affect the odometer.

Perhaps the easiest fix for a constant 2.8 mph offset would be to disassemble the instrument cluster and bend back the needle. I believe Ron Stygar did that to correct his Z3's much more optimistic cluster.

[pnosker]

Thanks for the advice. I think I'll leave it alone, I compensate for it on the fly (3mph isn't too hard to add haha). The only annoying thing was the calculation of the gas mileage that I was worried about. Thanks for the advice, but it looks either to difficult or costly to do.

[John Firestone]

The average fuel consumption calculations are often off, anyway, because of other factors. Your OBC's big, 18-button brother accepts a correction factor to make its calculations more accurate. I should correct mine, as it is about 5% too optimistic compared to the consumption I calculate from my vehicle log. Perhaps yours accepts a factor as well. In any case, the speed error experienced by the OBC should be the same as what you see in the odometer.

[pnosker]

Mine's about 19% off... haha.

[John Firestone]

19% happens to be what German VAT rises to next month. I consider that an error.

[weezer]

Pnosker.....most times it depends on the tire size. I wear a slightly off tire size with my K style OEM 16" rims: 225 45 16 and the speedo runs app. about 2.7-3.2 mph off. Had a different issue when using 225 45 17, the speedo ran 1.5-2.2 slower than actual speed.
Try using this:
http://1010tires.com/TireSizeCalculator.asp

[pnosker]

I'm running on the standard steelie tire spec. 185/65/R15
I had error with 205/60/R15s too on my alloys. It wasn't quite the same, but it's not tire size that's screwing it up.

[L84THSKY]

My average speed on my OBC seems off. I have driven to work on the highway at upwards of 75MPH and so far the MPH display never has gotten over 25MPH. Could this be because it is not getting it's reading from the speedo guage?

I was told by the BMW mechanic that my Cruise Control won't work, until my guage cluster can be read by the Cruise Control computer. Maybe that is a sign that the speed sensor signal from the speedo really isn't operating. If it was, both OBC and C.C. would have the signals they need.

Originally Posted by John Firestone

In any case, the speed error experienced by the OBC should be the same as what you see in the odometer.

[ATF]

L8,
You could always try resetting the MPH Avg while on the highway. Press the stalk to the MPH screen, then press S/R to reset it. Then test away.

[L84THSKY]

DUH......, that was easy enough. Why didn't I think of that?

I reset the MPH, and it works. Well, it is an average value, so I wouldn't swear by it.

I also did something very interesting today, I actually opened my Bentley manual and read it.

Hopefully, armed with some wiring charts, I can sort out the Fog Light issue, and maybe the Cruise Control.

Originally Posted by ATF

L8,
You could always try resetting the MPH Avg while on the highway. Press the stalk to the MPH screen, then press S/R to reset it. Then test away.