It’s All About the Tires
Vehicles in autocross are divided into classes along a spectrum from the least-modified vehicles (street, sometimes referred to as “stock”) to most-modified vehicles (modified, sometimes referred to as “unlimited”). Because they are so crucial to performance, tires are the most heavily-regulated modification in the sport, ranging from street tires to DOT-approved racing tires, and even non-DOT-approved slicks. Here’s how the Sports Car Club of America (SCCA) — the dominant sanctioning body for US autocross — breaks it down:
Street (Super Street, A Street - H Street)
This class category has the most restrictive rules, which keep competitors from feeling the need to make extensive modifications to their cars. Very few changes are allowed. Not even racing tires.
Street Touring (Street Touring Ultra, Roadster, Xtreme, Sport, and "Hot" Hatch)
These classes still require street tires, but a few bolt-on modifications are allowed to make the cars handle better.
Street Prepared (Super Street Prepared, A Street Prepared - F Street Prepared)
A few more modifications are allowed in this class, including DOT-approved racing-style tires. Some external engine mods (induction, exhaust, etc.) are allowed as well.
Street Modified (Super Street Modified, Street Modified, Street Modified Front-Wheel-Drive)
In these classes, your tires must still be DOT-approved, but R-Compounds are allowed along with mods like turbos, engine swaps, and wings.
Prepared (X Prepared, C Prepared -F Prepared)
Class rules get more intricate in this category, which is based on wheel-to-wheel road racing preparation for production-based cars. Non-DOT-approved racing slicks are welcome, and you can "gut" your interior to shave off weight.
Modified Category (A-Modified - F-Modified)
This category is for cars built specifically for autocross, with the most extreme modifications allowed. Road racing formula cars and sports racers are also welcome.
Karts, Classic American Muscle, etc.
The classes for karts, Classic American Muscle (CAM), Vintage cars, and College-engineering Formula SAE cars don’t fit into a neat category but are still an important part of many autocross events.
Find your autocross tire.