Haven't looked into it, but can you tweak/tune your cars? Or do you have to unlock better parts?
You buy upgraded parts with credits earned from racing. The street and sports level parts offer limited scope for tuning, but do offer performance upgrades.
Race level parts let you fully tune and adjust.
But you can always, no matter which parts are fitted, change minor things such as tire pressure. Also some cars come with parts that are street, sport or even race level already. And so you can choose those elements right away.
Upgrading cars can send you out of the class level you're supposed to be in. But things like suspension and brakes - which make a big difference - don't up your PI level very much.
One of the best upgrades you can do, if you can't afford to do the whole car, is upgrade the tires and brakes.
Another thing to look out for is if you do a total conversion - such as putting a completely different engine in the car. Any engine upgrades you've already bought will not be fitted to the new engine. Similarly if you change the car's aspiration. So if you want to fit a new engine or change the aspiration - do that then make any engine upgrades that you want.