The odometer stopped working on my 1993 850 and after about a year, I took it to the dealer to get replaced. The tachometer seemed to work fine and the speedometer worked within a few percent (based on "Your speed is:" radar guns and mildly obsessive calculation of speed on higways) when I dropped the car off.

When the car came back, the odometer was replaced and works like a charm, but the speedometer and tachometer are miscalibrated. When the car is off, both instruments read well below 0. In fact, the tach is so far off that the reading rarely goes above 0 unless we are on the highway. The speedometer shows readings 7-8mph too low across all reasonable speed ranges.

I took it back to the dealer and they insisted that the speedometer was working correctly (a dyno test elsewhere proved that to be incorrect) and that the tachometer must have been broken when I brought the car in. They want to replace the tach (~$750 part) and want me to pay for it.

I am going to get an estimate from another dealer and likely will have to sue the first dealer to get them to pay to fix the problem. I have a few questions. One, it seems to me that the "zero point" on both instruments is just set wrong. Is this something that can happen and how easy is it to fix? Two, what needs to be done to replace a odometer and how likely is it that these other instruments could be misadjusted in the process? Basically, I am trying to figure out what the heck they did to my car. Thank you very much.