Since the good stuff is locked behind CP and not cash, there’s not much you can do custom-tuning wise in career mode, so it’s easier to go with “auto upgrade” for the circuits… however.
A dumb thing this game does is it always “auto upgrades” keeping whatever parts are already on the car…. this means if you unlock a better option that you could refund CP back to get, “auto upgrade” won’t do that.
Tip: Always “reset to stock” in the tuning manager between races and THEN do auto upgrade. This can sometimes net in a 50+ PI rating!