Custom cars and tracks are contained within a zip file.
If it’s a track, it should contain two folders: gfx and levels.
Car zip files only contain a folder called 'cars'.

Windows

If the zip is packed properly and extract to the correct game folder it will prompt to replace/merge cars folder (custom cars) or gfx and levels folder (custom tracks)

Extract the zip files into your Re-Volt folder. Standard locations are
C:\Program Files\Acclaim Entertainment\Re-Volt
or
C:\Program Files (x86)\Acclaim Entertainment\Re-Volt

Linux

If you installed RVGL on linux, you will probably know where you installed it to.
Extract the zip file to your Re-Volt directory. After that, run the fix_cases script which is provided with the RVGL patch. This will make sure that all files are lowercase, which is necessary for RVGL. (You don't need to do this if you have Re-Volt running in Wine)

OSX

Never replace any folders, merge them!

Right click Re-Volt.app, click ‘Show Package Contents’,
navigate to drive_c/Program Files/Acclaim Entertainment/Re-Volt/
and copy the contents of the zip file into this folder.

Problems with incorrectly packed .zip files

It might occur that the structure is different. For example, if the zip only contains a folder with the car's/track's name, extract it to the cars or levels folder in your Re-Volt directory.

How to identify a track folder: Look inside the folder and see if you find lots of files where the filename is common with the folder name, for example petrol.inf. it is definitely a track folder.

How to identify a car folder: Look inside the folder and see if you find an Parameters.txt file. If there is one, it is definitely a car folder.