Speedometer is off by 10mph
 

I was wondering why my speedo is always off by 10mph.

If I'm really driving 40mph, it's reading 50mph.

I loosened the speedo on the cluster to install a new dimmerswitch, unplugged speedo and put it back in tight. Still does the same. But I noticed a bit less twitching. Used to sway/twitch alot, like by 0.5cm to each side of the speedo needle.

My records indicate that the transmission is not original but a rebuilt one. Could that be the problem? Is it possible to install a wrong speedometer?

Running on OE spec tires, 195/70/14. Tire psi doesn't affect speedo readings, currently running 33psi though.

1983 300D Turbo. How can I get the right speed/calibration? Thanks!!

Sounds familiar. Mine was doing about the same thing a few weeks ago, and I "fixed" it as follows:

http://www.peachparts.com/shopforum/showthread.php?t=131339

It's still working (about 1500 miles later), and I still don't know why it was off. I wouldn't recommend messing with it yourself unless you are willing to risk having to replace it. I figured I had nothing to lose, because it was pissing my off and I would have replaced it (or had it fixed) anyway. I assume I'll have to get it repaired correctly at some point, but in the mean time it's within 1-2 mph at 75.

are
 

you sure that is the correct size? it looks a little small to me. if so then it would result in what you have.

tom w

195/70-14 is stock for a 300D. I have 205/70-14s on mine, but that's only about 4% difference, not enough to compensate for 10 MPH. It's some kind of speedo magic I don't understand.