All your repaired cars you didn't sell can be tested on the test and race track. The next section focuses on this aspect of Car Mechanic Simulator 2018.
You can head to the Test Track at any time - you only need a car that is complete and working. you can learn a lot about a car's performance by driving around the designed track and stopping in the right place. Besides, it's cool to just try out a car you made yourself.
If you're looking for some entertainment then the Race Track should be more appealing to you. There, you can take any of your cars that are complete and working, and push them on the track. The lap time is saved, counted by checkpoints - you can cheat by cutting some corners.