Thanks Ron, that makes sense.
I can see how the car would look alot better without screws sticking out of the bottom. And I don't know what the clearance from the underside of the car to the track is either, now that I think about it. Maybe I'll hit a hobby store and see if I can pick up some small weights that can be dropped into some drill holes, then putty over them.
One of my co-workers also suggested to me that if you just use the wheels as they are, there will be more friction with the track too. He suggested sanding the wheels into something of "V" shape, so less wheel rides on the track. I don't know if that's ethical or not, or if it's a commonly seen thing, but I want to stay within the intent of the derby, so if that would be frowned upon, I'll avoid it.