The usual extra info...
There are 51 events (48 Trophy + 3 TT) and the rewards 25-50-75-100% are split 6-9-13-20GC. You will need all three cars for 100% and only the 370Z Nismo can be upgraded enough for the final six cup races.
With just the first car (350Z) fully upgraded you can get to 47%...
...and adding the second car (370Z) also fully upgraded gets you a little beyond 75%
To reach 100% I gather you will need to spend an extra 163GC on the 370Z Nismo (over and above the 166-25 net cost of winning it to begin with through its LTS at the end of the month).