use wadupdater or wadinstaller to put them into cars.wad file.
Or use gp4master to open a cars.wad file. Extract them into a folder and put all files in the cars/tyres folder. Then pack the directory back to a new cars.wad and copy the file into your gp4 folder.
That's all.
If you use gp4 v1.2 then put a decrypted gp4 v1.0 exe into gp4master folder and tell gp4master to use this file.
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