Just chuck the speedo cable in a drill, lock the trigger on and check it every hour or so until it's correct.
X2
i've done this before on an old car speedo, it worked, it just took awhile, because i didn't want to spin the cable TOO fast, and cause any problems! just don't pull Cam from ferris beuller!!!
Used the drill chuck method to see if an old speedo worked. Only got up to about 30 mph. Even at 100 mph it takes quite a while to change it by a few thousand miles.
If you have too many miles just run the drill in reverse (or the opposite direction of what it normally turns).
i don't know about reverse, the only way to check it is to give it a try! i'm showing almost 40k on my gauges, and after i do the rebuild, i think i'll keep it at the same mileage, it should be good for a few "wows!"