Let's say you want to add Swift to the list of possible chassis. Open up your DRIVERS2.TXT, find the NUMCH line, and change the number from 3 to 4. Add "Swift" to the end of the CHASS line. Change the DINFO number right after the car number of each of the Swift drivers to 3. Then close the file and set the read-only attribute. This is a necessary step. The problem here is that you are now no longer able to alter driver information in the game, and every time you exit the Driver Info screen you're told "Can't write driver info." The solution is to use Brad's driver randomizer - it creates a DRIVERS2.TXT from a DRIVERS3.TXT file which can be modified and made read-only without consequence. The DRIVERS2.TXT reads the DRIVERS3.TXT, performs its randomizer function, and writes a new DRIVERS2.TXT every time it's run. So this
system works like a charm - and hey, if you really want realism, you should be using a randomizer anyway!

Adding a new engine works the same way; however, for some reason, when a fourth engine (e.g., Toyota) is added, there's a peculiar high-pitched sound coming from the cars using that engine in the game. No one seems to have any idea why this occurs. I have never tried adding a third tire option (not that there's any particular reason to), though it works the same way; you can reportedly create new tire MIPs (textures in the car set's DAT file) for the third tire brand, but since I've never done it I'm not sure how they should be named. (Goodyears are WHEEL#.MIP and Firestones are WHEEL#F.MIP, where # is 1, 2, and 3.)