Great idea Turtle, but I think it might be most-useful for a speed race. In that case you could lessen the torque at the start of the race so the wheels don't spin out then increase the torque.
I'm not sure how such a transmission could be used when you're going for distance. In that case you want the least amount of string used (or movement of the weight) per distance the car travels. This would mean having a large drive wheel and a small axle. There is a point where the torque is so small that the car won't move, or it will start moving, but stop at some point down the track. You want to get as close to that point as possible without reaching it. Perhaps Turtle's idea could be used to assure that the car is always just barely moving, but never picks up any real speed or never stops completely. Maybe starting with enough torque to get the car moving from a standstill then lowering the torque so that it's always just barely enough to keep it moving would be useful—which is how the spindle is depicted in Turtle's pic.
~modest
EDIT: here's a link:
http://www.docfizzix.com/mousetrap-faqs.shtml
