Take a Vehicle,
SM are best, but anything that sits on the table should work.
Slit open the front. Glue a tape measure (the small type you get in xmas crackers are best) as close to the front of the vehicle as possible. Pass the lip of the measuring roll out the slit in the vehicle. Slit open the tracks (or whatever) and insert 2 wheel and axle kits from a model vehicle, smaller the better.
Now simply pull the taper roll out to the distance you're moving while holding the vehicle. Once satisfied, release the vehicle. It should roll to your finger.
Of course, this will only work on flat ground. A little bit of work, but you'll thank me for saving you the effort of lifting all those grams of weight.