This method will only work until May 20th when the servers are shut down.
Firstly you'll want 10-12.5million credits and buy the Ferrari F1 car either will do.
(if you don't have this many credits use a fast 650PP car in the Online A-spec Challenge No. 4 Nürburging track for 1Million credits a time.)
Once you have the Ferrari you'll be doing the 871PP Ferrari Formula Challenge on the London City Course (10 Laps). Before you start you might notice theses races have a 'Performance Difference Bonus' which means you earn more Exp and Credits for winning the race with a slower car.
Therefore what you'll want to detune your car as much as you can while still being able to win the race, my setup was to change the transmission final gear to 6.5 giving you a max speed of 164mph, Reducing Engine power level to 75%, putting aero to max on front and rear and adding 150kg of ballast.
This put my car at 800PP and for every win i got around 3Million credits and 470k A-Spec exp (the race takes under 7 minutes to complete 67kexp/min is the best I've found), you might earn more or less than this depending on your skill levels and how much you can downgrade the car yet still win the race, I would suggest you set the engine power to 85% and reduce it by 2.5% each time until you overtake first place on the last lap. You might be better off sticking at a level you can comfortably win every time but it can make the process more boring.
For the race abuse the AI as much as you can, use them for cornering and take advantage of how bad they are on the slow corners, and make sure you slipstream them at every chance because they'll be much quicker than you in a straight line.
(My exp and credit rate where based on the 200% increase bonus from logging in every day)