Adjusting odometer milage

Sometimes I need to replace the speedometer when referbushing older bikes. I usually get a used speedometer off e-bay. Is there a better way to adjust odometer mileage than using an electric drill and letting it run, sometimes for days. Are there small, reversible motors that run faster than a drill? It needs be able to both remove or add miles.

I'm not sure running the mechanism backwards at that speed is a good idea