Endurance racing, to be more precise. I was level 37 for ages until I decided to just do the Le Mans 24h race until I hit 40, it took 3 races. Ok, so it also took 3 days, but believe me, it's the fastest way.
You could also add some people from the Remote Racing friends thread in the B-Spec forum and share 3 drivers while doing the 24h races with the other 3, that way they'll likely use your Bobs while you do something else (like work, sleep, etc) and earn a bit of extra XP on the side.