What is a PAX?

A brief explanation of PAX

The PAX is a way to compare times for drivers of cars that are in different classes. It does this by “normalizing” everyone’s times to the fastest class. PAX stands for “professional autocrosser’s index”, and is also called RTP, or “racers theoretical performance”. But, over time, it’s sort of become it’s own noun and verb. (some of the best words are that way!) You say “PAX” around a group of seasoned autocrossers, they know what you’re talking about.

This idea has well-established roots in the SCCA Solo community, and the indexes are updated annually, usually sometime after the Solo Nationals in September. The index for each class is a multiplier for your run time. Multiply your time by your index, the result is your theoretical time had you been driving an “A Modified” car, which is mind-alteringly fast. If you’re in an A Mod car, your index is 1.000! If you’re in one of the faster stock classes on street tires, your index would be a little over 0.800. Yes, A Mod is that much faster!

Some people hate PAX, some people love it. The problem with it is that for it to be accurate, your car and the cars you’re comparing to must all be “a car that is highly competitive in class”, and fully prepared to the absolute limit of the rules. If you go to the Solo Nationals, you might see a lot of that sort of thing. But, for a casual local autocross? Yeah, we’re driving the car that we have… and it’s probably not fully prepared. But, most of our competition is in the same boat! So, the PAX is still a reasonable indicator of how you and your car are performing compared to drivers in other classes. It actually works a lot better than we expected it do. The faster drivers always float to the top, and even more so if they’re in a competitive and well-prepared car.


FAST uses different classing from the SCCA classing that the “official” PAX, so we’ve used the PAX indexes to come up with a “FAST PAX” to suit our specific classes and preparation rules. We update them at the beginning of each season.

For future reference (ours, not yours), here’s how we derive our indexes:
Average PAX index for all of the classes that make up a given FAST Class, Add 0.020 to all Mod classes, then subtract 0.010 from M4 and S5.¬†For R1 we use the index for SSP (Super Street Prepared), which is about the fastest “streetable” race tire class, and sort of inline with most of what we see in R1.

This is the current FAST PAX:

R1 - 0.853
M0 - 0.839
M1 - 0.829
M2 - 0.822
M3 - 0.805
M4 - 0.794
S0 - 0.819
S1 - 0.809
S2 - 0.800
S3 - 0.802
S4 - 0.785
S5 - 0.770

How We Use PAX

You’ll find a sheet of PAX Results published for almost every FAST event, and they’re usually just included for fun. You want to know how you’re faring against the drivers in faster (or slower) classes? There ya go. We don’t do anything with them, they are just informational.

If we opt to include a “PAX Class”, then it would be included in the Results By Class sheet for the event. If you register in that class, then we would use the appropriate index for your car to determine your PAX Times and score your class accordingly. (fortunately, this is a fairly automated process!)

For the 2020 season, we are not doing any official PAX Classes.

Updated 8/2019