Speedometer is WAY off

#1

I posted this on bf.c, but thought I’d try it here too.

I recently replaced the odometer gears in my '87 325is and reinstalled the cluster. I’m not sure if it started happening immediately after that because I didn’t drive the car much, but now the speedo is reading really high. Not the usual 4 - 5 mph high, more like 20 mph high or more. At 45 mph, based on my GPS, the speedo is reading 65 or more.

While this may do wonders for my lap times,:laugh: I’d like to get it fixed.

I removed the plug at the diff and sprayed it with electric parts cleaner but that didn’t help.

I don’t think the odometer gears are the issue. The odo is still accurate and I think they are driven by a different method than the speedo.

Any ideas?

#2

The odometer gears shouldn’t affect it. What wheels/tires are you running?

#3

I think the issue is most likely in your speedo or instrument cluster.

I just replaced my speedometer a few days ago after have after having strange issues with it being intermittent. My speedo worked about 5% of the time and my odometer didn’t work at all. Then one day my odometer worked and my speedo didn’t. I replaced the gauge with a spare and now the speedometer works fine.

I instrument cluster also has a white plug in the front which I believe tells the guage cluster what kind of car it is in, so if you swapped a cluster with a 525 or something it could possibly affect that.

#4

Replacement of the odometer gears has nothing to do with the accuracy of the speedometer. If the odometer isn’t showing the same error, the problem lies in the speedometer since both get the same signal from the sensor in the differential.

#5

I don’t use my analog speedo at all. In fact I rarely look at speed unless I’m trying to work on something specific that I saw in my post session data analysis, and if that’s the case an analog gauge is no meaning (no enough resolution at a quick glance) so I’m reading it off my digital display (DASH3) hooked up to my data logger (DL1).

So I’m saying I wouldn’t spend any money trying to make your stock speedo accurate.

#6

look for the small print on the face of the speedometer for the “K” number. That has to match your old speedometer.
bruce

#7

Thanks for the suggestions. This is a street legal track car, so a working speedometer would be nice.

To clarify, this is the original speedo and gauge cluster that came with the car when I bought it. Factory original as far as I know. I just pulled the cluster to replace the odo gears and reinstalled everything. Stock basketweave wheels, with same tires as when I bought the car almost 2 years ago.

Right before the odo gear job, I replaced the trailing arm bushings, so I was working around the diff during that process. I drove it some after the TA bushing work, but not much, so the speedo may have been off then, but I don’t think so.

I don’t think the odo gear replacement itself is the cause, but maybe something else I did when I had the cluster out, or possibly when working on the TAs.