It sounds like common sense, but every year I see cars that don’t fit on the track. In order for your car to race, it needs to be able to roll down the track. If there isn’t enough ground clearance between the guide rail and your car, your car will scrape the rail and slow down. If there isn’t enough space between the wheels and the guide rail, your car will slow down. If you build a tricycle, one wheel will ride on top of the rail and your car won’t run straight.
Check your rules out. They probably tell you what your car’s dimensions need to be.