Speedometer Drive

Not sure what was in it originally. The car was a /6,auto,7 1/4 rear end.

I am putting in an electric speedo that shows kms.

An a727 is going in and I will be using an Auto Meter speedometer sender unit # 5291 so I can dispense with the usual problems of tire size and number of teeth.

With this unit, you drive a specified distance. You press a button at the beginning, drive the distance at any speed you desire and press a button at the end. The unit does the rest. Change tire size, repeat above.

I was having nightmares trying to figure out what tooth number I would require with my set up and have it read correctly on an analog speedometer that reads kms..

So you need the length of the gear shaft...what is short / long ?

Thanks
ian.