We used to have some old speedometers laying around when I was a kid, and we would run them off of the tire on a bicycle (turned upside down). It might be faster to run it forward - depending on where it's at now. If a drill is your choice, and if it is too slow, you need to build some sort of bracket to hold it against a rubber wheel that you've attached to the drill (like in an arbor for a small grindstone, or wire wheel). Set the end of the speedo against the wheel, and turn it on.
Taking it all apart would also be an option, but you'd just need to pull the odometer part of it, turn the wheels back to zero, then put it back together. There may be some pretty small parts that want to go all over the place when you take it apart, so I'd try the other method first.
