Guys,
Your advice to the den mother about how to help her son win the Cub
Scout derby was all screwed up. No wonder you didn't pass physics.
You were only half right when you said that all bodies fall at the
same rate regardless of their mass. This is true only in a vacuum.
(Like space, not an Electrolux. All Electroluxes do fall at the same
rate, regardless.) 
Outside a vacuum what matters is the
ratio of mass to surface area. The car will go fastest if it is both
heavy and aerodynamic. It should weigh the full 5-oz. limit with as
little frontal area as possible. Ideally the car body should be
shaped like an elongated tear drop cut in half lengthwise. The axles
should be placed high on the body so that the bottom of the car is as
close to the track as possible. Finally, a series of tiny dimples,
like those on a golf ball, will add a little aerodynamic boost.
Or, on the other hand, she could just back off and let her son have a good time with it, even if that means losing to all the other kids with overbearing, interfering parents.
Keep up the good work, guys!
Aaron Howald
