Get in a car. Go to the car's dealer. Go to the Tune Shop. There you can add parts.
However, if you want to tune the parts you have, it's an icon with all the others just before starting a race, towords the right side. The icon is a red tool box. You can also click on the start button when at the garage.
aznwonderboy - Theat's 2 threads you've created on exactly the same subject. You could have found the answers by looking round the site. Please don't create any new threads until you've had a look around first.
