You can do many XP without boring and unnecessary grinding.
Go to Seasonal Events and do EVERY seasonal race available.
You will have many great races and amount of XP will be enormous once you finish them all.
dupakI finished them all.
There's no pot of gold at the end of the level 40 rainbow...
Seasonal Events -> Lightweight K -> Eifel (kart)
50000xp in under 3 minutes.
2000000xp/1hour
NeocodexDelorean + many giggawatts + 88mph
red bull vettel challenge.
about 2mins each time trial. 150,000,000 exp each, 3 in total.
Thought it's a one time thing. Still worth it.