The scope of this article is to provide information regarding the numbers and class letters that you must display on your car during a Solo Event.
Let's start with the official SCCA 2012 Solo Rules:
3.7 VEHICLE IDENTIFICATION
A. All vehicles must display numbers and class letters on both sides,
which must be readable by Timing & Scoring, Course, and Grid
workers at all times.
B. Only one set of numbers and class letters may be visible while the
vehicle is running.
C. Class shall be represented by the upper-case abbreviated form rather
than be spelled out. Ladies’ classes shall be indicated by the letter
“L” following the class letters. (Example: “BSPL” instead of “B Street
Prepared Ladies”).
D. Numbers and class letters should be positioned next to each other.
All letters and numbers must be on body panels, not on windows.
All numbers and class letters must use the same typeface and the
same color, and this color must provide adequate contrast to the
background color (see example).
E. Numbers must be a minimum of 8” high with a 1.25” stroke. Class
letters must be a minimum of 4” high with a 0.75” stroke. In all cases,
the height of the class letters must be between 25% and 75% of
the height of the numbers. Stroke width must be at least 10% of the
height (see example.)
F. The “1” on two-driver cars and the “L” on Ladies class cars are subject
to all of the above requirements with regard to placement, color,
size, and stroke.
G. Karts may use numbers and class letters of reduced size provided
that the following conditions are met:
1) Numbers must be displayed on the front and rear in addition to
both sides.
2) Class letters must be on both sides.
3) In no case may the numbers be smaller than 6” in height with ¾”
stroke, using a high-contrast color and background.
H. For National Championship and National Tour competition, current
official SCCA® required decals must be displayed on each side of
the vehicle in a prominent location.
For National Championship and National Tour events, one (1) official
SCCA®-approved National sponsor identification logo must be
displayed in an upright position, in a prominent location on each side
of the vehicle. Additional sponsor and/or event specific decals may
also be required; refer to event supplemental regulations. Further
information is contained in Appendix F.
OK, now that you are totally confused, let's discuss what that all means.
Basically it means that you should be able to see your numbers and letters at a distance, while your car is at speed.
Check out the following examples:
GOOD 
BAD 
UGLY 