Find a common lp turntable...it will have a turntable with a diameter of around 12 inches...maybe 11. Something like that. It ought to have a rubber edge on it. Most of them did. Hold the "wheel" made by the outside edge of your speedometer as shown below...the spinning part (which should be a diameter of around 3/8") against the turntable (set for 78 rpm speed). Assuming there's not too much internal drag, your speedometer shaft will be spinning at 1000 rpm or very close to it. Good enough to let you know if you're wildly off or reasonably close. I can offer plenty more advanced math but that's as far as I can go quickly this afternoon. Good luck.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
