When cutting a Pinewood Derby car, you want to make sure that the wheels are perfectly round and balanced, with the axle hole dead center in each wheel. As a former Cub Scout who has many fond memories of building pinewood derby cars with my father, here are some thoughts that may help. Let's start with the basic fact that pinewood derby cars roll down a track under the influence of gravity. Imagine that the tires were made of soft, sticky rubber -- tire stickiness would be a big source of friction. I am going to guess, however, that axle friction is lower than aerodynamic drag created by lift.
Ignore the track for a moment -- let's say I take two pinewood derby cars and I drop them in a vacuum.

Since you have no control over the tires, you can ignore stickiness (and other factors having to do with the tires like diameter, rotational inertia, etc.). Therefore, you would want to create a car with the lowest aerodynamic drag possible and work to drive axle friction to zero. We have all seen (or heard of) the experiments where a feather and a heavy steel ball are dropped in a vacuum. Let's say that you could design a car so that it actually flew; with the wheels just off the ground, axle friction would fall to zero. Here's why: The force acting on the car is calculated by the formula Force = Mass x Acceleration.
Any wing that generates lift also generates drag in the process (see How Airplanes Work for details).

Therefore, if a pinewood derby race involved dropping two pinewood derby cars in a vacuum, it wouldn't matter what the cars looked like, how much they weighed or anything else -- every race would be a tie! You also want to make sure that the wheels are perfectly round and balanced, with the axle hole dead center in each wheel.
Or you would have to build a number of cars and see how they performed compared to each other. This force is competing against the force exerted by aerodynamic drag, so you want as much mass as possible to increase the force.