You’ll have to provide a bit more information to get meaningful answers 
To eliminate a few possibilities, turn on stat unit and show is the numbers when FPS is bad.
Assuming you have some more advanced cars ready already and not just cardboard box racers:
- How are your cars set up? Do you do any funky raycasting for custom logic of the wheels?
- How many polys do they have?
- How many are on screen?
- How many material instances do you have per car?
- Are you using the track generator tool?