Next in importance is reducing wheel and axle friction. This is accomplished by applying a dry lube to the areas along the axle and wheel bearing points. Also you may want to remove any burrs that might occur on the axle or wheel itself. One can carve out the inside of the KubKar and insert metal washers or bolts to bring the weight of the car to its maximum. 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.