What happens if you go in a race with a car which is too quick?

[Harsk100]

What happens if you race with a car which is too quick?

It says:
Your Impreza is too quick for City Limits, for example.

 

[The Stig Farmer]

You car is automatically modified (downgraded) to a lower PI.

If it can't be, then you'll need to try with another car.