I helped my boys polish the nails used for the axles, talked physics a little and let them have at it. One boy asked for help getting the wheels as close to parallel as possible. both boys had screaming cars that took their divisions. I agree with all the comments above.
If you look at it from a physics point of view, the car with the most potential energy should end up with the most kinetic energy if the friction is equal. Since the car starts with its rear in the air and ends level, the more weight in the rear, the more potential energy. One of my boys skeletoned out the car with a drill from the side and drilled holes in the rear to accept bullet slugs. that car looked like it was propelled by 9 rocket engines. (hollow points)

We did have to relocate the rear axle as when he put it down, the front wheels came off the ground, too much weight in the back.
That car was a screamer but after it fell off the table and bent the front axle, it ran faster. Highfinance is right, three-wheelers run really well.
Sometimes it is hard to watch them do something goofy, but worth it to watch them learn.
I was fortunate to have my two oldest boys in my troop when they got their Eagles and can't say enough about the entire scouting process, cub up.
Any Dad who helps his cub in whatever capacity with that car and scouting in general deserves kudos for spending time with and helping out that kid IMHO.