All programs help you editing the RES car files, i.e. they let you change a car's performance and behaviour, but not its appearance. A very sophisticated editor for DOS that was programmed by Markus Nagelholz. It gives you complete control over the car data file.
Some people find it hard to use but it is the ultimate tool for any serious car editing.
An easy to use DOS based program that lets you change all basic car features like gear ratio, maximum RPM and alike. A great car editor for Windows written by Mirko Ruschmeyer. It also offers a visual power curve. It enables you to set the maximum speed an opponent 'dares' to drive fall all the track tiles separately. The program provides a fancy, easy too use interface and is mostly self explanatory.