Thanks to dedicated SCCA Member and statistician-extraordinaire, Rick Ruth, the 2012 Pax Indexes have been set and released to the public. For those unfamiliar, The Tire Rack does a fair job at explaining the purpose of the Pax Index:
"The PAX index... is a commonly used handicapping system for comparing times of cars which are not in the same class. The index is composed of data from across the country, and tries to take the driver out of the equation and compare only the cars. These indices are based on cars prepared to the limit of the rules and driven by top drivers... The index is multiplied against your run time to provide the time you would have had if you had been in an A-Modified car. It is interesting to compare the index of one class to another to see how much time you should gain if you prepared your car to, for instance, Street Prepared rules. Look in the rule-book to find out where your car would be if you made modifications. You can also see that alphabetically, higher cars are not necessarily faster."
So with the latest 2012 figures posted, you can start planning your build now. Two big Pax "unknowns" were the new Street Touring F (STF) class, which had no data to base the index on, and the new ASP, since it absorbed the BSP Mitsubishi Evolutions and purged itself of the Corvettes. The STF index appears very light at first glance, dubbing STF officially the slowest class in solo.