It's very fiddly in my experience. In the game set up menu , on the first tab set car class to any. On the third tab there is options for minimum and maximum pi ratings. Set these as desired. Then press to accept and you should be shown a list of cars taking part. Click on each individual car and you will be given an option to change that car. The more specific you set the options on game set up , the less choices you will have and the easier it will be.