How many lanes are on your track?
Are you using a timing system?
If yes, your software may dictate how you conduct the race. One way is to number the cars from 1 to X, where X is the number of cars to run.
Each car gets a turn and cycles through each lane. Once it has run in the all the lanes, the next numbered car is added to the group and the car that has run on all lanes is "retired" to the pit/garage area. We used garage sale stickers for the bottom of the cars so that we did not mar the paint job or decals on the cars.
If you are not using a scoring and timing system, the following site has some downloadable spreadsheets that they site owners have been kind enough to share, and might work for you. They are handy for Space Derbys too. Plus, I think they have something for a Raingutter Regatta.