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!!