Hi Len,
We used to make ours as light as possible and then add lead weight to the bottom of the car to bring it up to the legal weight. The weight down low seemed to make the car faster. The boys and I made one that looked like a UPS truck.(I was a UPS driver at the time)I don't remember if it was legal to add graphite lubricant to the axels, but that would also help. We lived in Dearborn, MI at the time and had some parents that worked at Ford design. Needless to say the pinewood derby cars were beautiful, but I don't think the kids ever laid a hand on the building of them. As mentioned the axels are the most important item. This all happened 30 years ago. Hope this helps some.
Pat