The only editable ones are either very old (Papyrus Indy Car Racing 2),
or mods to moderately old PC racing games (Grand Prix 2, Grand Prix 3, Grabd Prix 4, NASCAR Heat, F1-2002, F1 Championship 99-02, and NASCAR Racing 2003 Season).
Indy Car Racing 2 has a fair number of CART and IRL carsets, but very few addon CART or IRL tracks, and the game probably will not run under WinXP.
Grand Prix 2 seems to have the most CART, CCWS, and IRL carsets and tracks.
Grand Prix 3 probably is second in that respect.
Grand Prix 4 has a CCWS carset and only maybe a couple of CART/CCWS or IRL tracks.
NASCAR Heat has 2000, 2001, and 2003 IRL carsets and many IRL tracks (as they're largely the same as NASCAR tracks).
F1-2002 has an IRL 2002 carset and a number of IRL tracks.
F1 Championship 99-02 has a completely stand-alond CART 2000-2005 mod, which includes most of the cars (with some major errors in their liveries in a few cases) and most CART 2000-2005 tracks, that is downloadable for free and does not require the original game in order to use it (although you will be missing the a Montreal track if you don't have the original game).
NASCAR Racing 2003 Season has IRL 2004, IRL 2005 and CCWS 2004 carsets and most IRL tracks (as they're largely the same as NASCAR tracks), and a few CART-CCWS tracks. It does have the slight problem that all the carsets, including the CART one, use the one same IRL carshape, and the major problem that all of the cars use the original game's "Trans-Am" physics, which means that the cars are a bit too slow and their physics are meant to replicate a 3000+ pound car.
The old MicroSoft CART Precision Racing never had any addons that I ever encountered, though, oddly enough, a number of its tracks were converted to other games, even though no new tracks were ever made for the game itself. The game suffered particularly from allowing a maximum of only twelve cars in a race.
The Indy Car game released for PC a couple of years ago has proven to be completely uneditable with absolutely no addons that I know of, and more than a few faults out-of-the-box. (Would you believe that it doesn't have any rear-view mirrors?!)
Edited 2 time(s). Last edit at 07/23/2006 08:06AM by JackiMatra.