That was so fun when my boys were young. Used to put the "axles" in the drill and take off any burrs and polish a little. Most important was to put a plate on the bottom with 2 screws and be able to add weight under it for the maximum allowed weight. Graphite on the axles and make sure all 4 wheels sit on the ground and are lined up. The more times the car goes from side to side against the guide the more drag on the time. Areodynamics helps but an overweight block can beat a lighter car that's done well. Check the wheels for mold lines and remove any you find. Good hands-on learning expierence for boys. That's my 2 cents.
Here's a quick video of the wheel truing...........0.004", that's 0.002" out of round. Shaft videos if you're lucky. Five in our den came to the shop for a "build" night and I could only get one father/son interested in the chucking the axles up in the mill and de-burring and polishing them.