The car setups included in the game strike a balance between being fast and fun to drive, while still being controllable for most players. If you want to gain more of a competitive edge, you may wish to make your own car setup.
Access the garage in the at-track menu by selecting "Change" on the Car Setup box, selecting Custom Race or Custom Qualify, and selecting "Edit". If you don't see the option to edit your car setup, make sure your driving aids are set to "Hard" or higher.
The garage can be intimidating; there are a lot of settings to change! Thankfully, there are plenty of users in the Tony Stewart's All-American Racing community who provide helpful tutorials on making effective setups. For instance, @RED4424 has put together a series of videos on how to make setups for specific tracks. Check them out for help!