Derby Car Numbering System

I use colored circular spot labels made by Avery.
Each age group is assigned a different color.
I prenumber the labels, using a different range for each age group. (Typically I have less than a hundred cars in each group, but if you are going to have more than a hundred cars in a group, use 1000-1999, 2000-2999 etc..)

Group 1 100-199 Yellow
Group 2 200-299 Blue
Group 3 300-399 Purple
Group 4 400-499 Green

At registration, there is a column on the registration form for the car number.
As cars are registered, I write the number from the sticker. Typically there is a group of numbers for each age group for an outpost, so I cut out the whole group of stickers together and give them to the senior commander. (When the next outpost registers, I look at the sticker sheets to tell which numbers come next. This prevents the duplicate use of numbers.)

After all the cars have been assigned on the registration sheet for a single outpost, I make a copy of the registration sheet and give it to the person in charge with the instructions to place the sticker on the rear bumper of each car. (This helps the starting line judge to properly position the car.) The sheet shows which number goes with each car.

This system works well with computer timing systems too, because each car requires a number assignment.