We've all shown up to an event to find another car running the same number in the same class. Be prepared with these number segments. Arrange them to make any number desired. They have a 1.25" thick stroke, so they're legal for SCCA solo.
Overall height depends on the configuration you choose. A "1" is 8" tall, and an "8" is 9.5" tall.
Simply select the color and class you'd like. You'll get a set of class letters for each side of your car, and supply enough number segments to make any number (7 segments per "number"), and deliver them to your door step.