[TAGTOboy]

Originally Posted by johnta1

I think you can get an adapter to go between the cable and the tranny.

But otherwise just remember to change the mileage for MPG calculations by 21% up/down.

This was absolutely true ( may still be available).
Back when I worked at a number ofPontiac dealers, we set up rearends with gearing changes, then took them to Speedometer Exchange. They repaired/ replaced speedometers when necessary and calibrated them with an adapter that was little more than a housing (which would fit in a large hand) with the appropriate gear sets to correct speedo. They used to do this for law enforcement also.