(THIS METHOD DOES NOT WORK ANYMORE AFTER THE GT5 SERVERS HAS BEEN SHUT DOWN)
Requirements:
1. Internet
2. Super GT Car (Recommend NSX Super GT)
3. 550 Performance Points
4. Seasonal Events (Super GT Events)
5. 200% Bonus
Every 6 Minutes you get 2,000,000 for coming in 1st place with the requirements above. If you don't have 200% Bonus you could still get credits but not as much as with a 200% Bonus. I list the possibility credits you could get with getting in 1st and setting your car to 550 performance points:
110% Bonus = 1,100,000 Credits
130% Bonus = 1,300,000 Credits
150% Bonus = 1,500,000 Credits
170% Bonus = 1,700,000 Credits
200% Bonus = 2,000,000 Credits
Hope this video helps you in getting fast credits. Enjoy. :)
