Calibrate a Speedometer

[69Ford]

I worked over a speedometer "wish I would have taken pictures" to convert a early 60's speedometer to work with a late 60's mechanism. I can see how to calibrate it but need someway to spin it at the right rpm to mph. Anybody ever done this. Anyway here's a pic of what it looks like.

[Ranchero50]

Looks good. I always thought 60 rpm was 60 mph. I'd try a battery powered drill on low speed (if you can find one that spins below 100 rpm and compare them. You can also run the truck at your 60mph rpm in fourth with the speedo hooked up.

Couple different ways to skin that cat.