The usual extra info ...
There are 51 events (48 Trophy + 3 TT) and the rewards 25-50-75-100% are split 10-15-21-33GC.  You will need all three cars for 100% and only the Porsche can be upgraded enough for the final two cup races.
Afraid I didn't note down the PR for 75% but you can certainly reach it without the Porsche, and probably with only the Sesto, or only the Vulcan, as long as well upgraded.  With both, and the Vulcan fully upgraded, you can get this far:

To do the final two cup races you need apparently pattern 5343343 on the Porsche, a further 380GC (new lowered costs!) more than the initial cost of winning it...