What drives the electric speedo on an early 80's 911?

[Cairo94507]

I am changing from my original 914-6 speedo to a 77-88 911 electronic speedo. I will gain the extra digit on the odometer as my car has 89K original miles and I do not want it to reset to all zeros when it hits 100K after it is back together. Stock, my speedo was cable driven. Where does the electronic speedo get the speed signal? How do you calibrate it? Thanks for the info everyone.
Michael

[rwest]

There is a plastic ring inside the transmission with a bunch of magnets on it and the sensor that is mounted to the side of the transmission and wired to the speedo "counts" them.

[Matt Monson]

See the attached picture.


Your 914 gearbox has no allowance. What I would do, if it were my car, would be to photograph it just before it flips over and after, along with time date stamps on the photos. Add it to your documentation on the car. There are far more instances of someone trying to pretend that a car with 128k mi on the odo really has 28k than the other way around with proper documentation as the mileage goes up.

[Cairo94507]

OK, let me see what I can figure out. Thanks for the photos and info.