Well, after the discussion in this thread
, I decided to try recalibrating my speedometer. Did it last night and figured I'd post a brief write-up.
First off - check out this thread
on e30tech that explains the concept of how to do it. I essentially followed the procedure used there, except instead of setting up a signal generator, etc etc I just did a trial and error method.
If you can pull a cluster, this procedure is not hard at all. See pics on e30tech for what the speedo looks like out of the cluster. To figure out your real speed based on the tach, use a gearing/speed calculator with your tire size, transmission and diff ratios to figure out what RPM corresponds to what speed.
1) Pull cluster.
2) Remove speedo from cluster.
3) Measure resistance of pot resistor using multimeter (optional, but useful)
4) Decrease resistance (adjust clockwise) to make the speedometer read lower for a given input signal. Only adjust a little bit at a time - my total adjustment was less than half a turn.
5) Re-assemble and test in car.
6) Repeat until accurate.
In my car, the resistor came from the factory set at 22k ohms and the speedometer read about 10% high. I ended up with it adjusted to about 10k ohms to read about 2-3% high (I wanted to leave a bit of a buffer - and I got lazy in adjusting it