Unlock The Delfino Feroce by completing Kudos World Series on Steel or higher.
Unlock the Ferrari 250 GTO by completing Kudos World Series on all Bronze medals or higher
in the car selection screen press right till you get to the garage press A then press Y when you are in the garage you can now walk around go over to the arcade machine by the radio and press A
Unlock the Mercedes CLK-GTR by completing Kudos World Series with all Gold medals or higher
Unlock the Porsche 911 GT1 by completing Kudos World Series with all Silver medals or higher
This will only work if you can store your original game on the HDD of the Xbox.
FTP onto your Xbox and find where you have stored your game.
Now in the PGR2 folder there will be a folder call "car dynamics".
Now make a backup of this folder (just in case).
To get any car on any track just do the following.
Find a car that you can use on your chosen race.
Find a car that you want to use.
Now rename the file of the car you "can" use with the one you "want" to use.
If you like you can open the file in a text editor and actualy change the handeling,speed, spec of the car.
The car will show as the original car but will have the dynamics of your chosen car.