The usual extra info...
There are 51 events (48 Trophy + 3 TT) and the rewards 25-50-75-100% are split 7-11-15-23GC. You will need all three cars for 100% and only the Audi can be upgraded enough for the final three cup races.
For 75% you'll need one of the cars upgraded to just PR42.5 or thereabouts - tiers open up well in advance so you can get there with non-cup events :
The Audi at PR53.4 from winning the LTS will do two of the last three cup races, but needs further upgrading for the final one at PR56.3.