- Improves visibility for timing and course workers
- Better choices of contrasting colors
- Makes your car look super racy
Numbers must be a minimum of 8 inches high with a 1.25-inch stroke (thickness).
Class letters must be a minimum of 4 inches high with a 0.75-inch stroke (thickness).
There are many vendors of magnetic numbers. Here are a couple in our neighborhood with a consistently high quality product:
Solo Performance Specialties - St. Louis
MiDiv Solo Series supporter
http://www.soloperformance.com/

iZoom Graphics - Tulsa
Run by area autocrosser/RallyCrosser
http://www.izoomgraphics.com/

A basic set of black or white numbers and class letters is relatively affordable. You can also spice up the look with colors, different fonts, and outlines for additional cost.
Local sign shops can also make these for you, or you can cut your own from a sheet of magnetic stock. Magnetic-backed vinyl can be cut with scissors or a sharp blade, but the results often look less than professional. Also be wary of thin, low-quality magnetic material from a craft store. Good material or nice pre-made numbers will stick better and last a long time.
Magnetic numbers can curl if stored haphazardly and will stiffen in colder weather. A warm metal surface (hood of a warming car) can usually get them to flatten out and remain pliable.