If you just want to change the performance, follow this steps;
Method 1= Use TDUF for changing performance of the car. TDUF doesn't work on some people's computers, if it doesn't work for you, follow the second method.
Method 2=
1-Use TDUPE (Included in Platinum Toolkit) for changing performance of the car, then save 1 time. After save, because tdupe is an insufficient program, it will save car count incorrectly and the game will not open.
2- Then open TDUMT program(Included in Platinum Toolkit), Click "File Browser", go to database/db.bnk and find the "tdu_carphysicsdata.db" inside of db.bnk file, then extract "tdu_carphysicsdata.db" to desktop.
3-Find the program TDUUT in this forum and download, Drag "tdu_carphysicsdata.db" file onto TDUUT's exe file. TDUUT will convert this file to hex file. Open the hex file with notepad or notepad++ and find the line that says "// items", (I'll put a picture of it) change the item count to 1110 and save the hex file.
4- Drag the new hex file (tdu_carphysicsdata.db.hex) onto TDUUT's exe file. TDUUT will convert this hex file to db file.
5-Go to TDUMT program and change the "tdu_carphysicsdata.db" with your new db file. And thats it.