Old speedo wire section ( one of the ends... both work the same ) between drill and speedo.

Then run the drill for 5 minutes ( I couldn’t test for more time because my drill began to heat ) untill get the speedo set and matching with odometer.

At correct speed 5 minutes the odometer should get 2.75 miles with a 550 rpm speedo at 33 mph aprox. Make your maths with your drill speed.

If you drill doesn’t heat, could run the drill for more time. I made this several times untill match the best I could.

( this is a Kilometers speedo, that’s why you notice the diff scale ).

if you find two drills with diff speeds, could make the test at diff speeds. I just made this because is the only one I had and nobody around had one with a known speed spec and reverse speed feature.