There are a lot of gadgets out there to do the job but they all rely on the power of the motor to provide the rotation. This means that you are taking life off your motor and more importantly the gear, you have to be really careful. I saw Paul Darby (AKA The Guvernor and winner of T.V. series `Short Circuits ) do a much better and quicker job with a Dremel drill (£8 at B&Q on offer) . Insert the axle into the drill and power away with one wheel and tyre fitted at a time and sanded on a flat board with good sand paper stuck to it. Now you can get your tyres down a lot lower (great for magnet cars!) and no wear on the gear! Same idea for the front tyres and the guide is gonna sit lower in the slot on most cars.