Some feel that if one places the car weight to the end of the car, the stored potential energy in the race will make a faster car. Lubrication is the key in many designs. Powdered Graphite is one type although there are many other liquid lubricants that may gain an advantage. One can slightly file or bevel the bearing surface of the wheel to reduce the amount of friction touching the car surface. It is strongly recommended that one refrain from using weights which contain toxic levels of lead.
Details are important.
A high gloss finish. The length should be suitably long enough to allow the cars to accelerate and maintain speed. Some cars start fast, others maintain speed better. The best cars do both. The starting gate should be made so that all cars start at the same time.