I did forget to mention... make certain your wheels turn FREELY. Ever do a Cub Scout soap box derby car? remember how freeely the wheels turned on the ones that went fastest? You want your model's wheels just as free turning if possible.

If you have threaded screws as your axles.. it helps to drill the wheels out and put brass tube bushings in the wheels if you have plastic hubs. The plastic grabs the threads and that makes the wheels catch. The brass won't catch. (make sure you de-burr the ends...)