I find it today and i was surprised that no one mentioned already.
Start a rally event with 1 star. Find a very short event under 2 mins. When you finish the race do not press A to continue but instead press X to race again. This way you have 2 BIG advantages.
1. NO loading time for the stage as you play the same stage over and over again
2. Every race is under 2 mins (i found a race in USA with an average time of 1:45 and managed to do 1:36 almost every time).
You progress for the 3000 km achievement also.
Only disadvantage: you cant collect the money for each race as you restart the race. But if you want to collect almost 5 to 6 millions in order to buy all the cars the best way is to race gym khana championship events.