The most effective method of all right now is one of the weirdest, and it involves an aspect of the game that really doesn’t show it at its best.
You’ll need to unlock the Fishermans Ranch rally circuit, a fictional special stage which seems to be set on the border between the USA and Mexico in Arizona, by
completing Menu Book 36. Ideally you should also have a Gr.B rally car too, which you can win from either
Menu Book 37 or International License A.
With that done, apply some Dirt tires and head into the Dirt Champions race. It’s rated at three chillis, or “Hard”, but there’s no PP limit, so you can go nuts with the tuning if you wish.
While the race only grants 65,000cr for the win — rising to 97,500cr with the hard-to-lose Clean Race Bonus — it’s only a single lap event. If you can deal with the motion sickness induced by the cars lurching over the jumps, it’s possible to complete the race in less than 3:20.
That equates to almost 30,000cr per minute of racing, and if you’re quick enough mashing the X button to advance through the menus, 1.75m credits an hour.