You need to buy a Chevrolet Corvette ZR1 (C6) ,09 then go straight to GT Auto and change the oil.( It increases it's BHP ) After the oil has been changed go to Race Modification in GT Auto and Race Modify it. Once Race Modified go to the Parts Shop and buy everything you can. Now you need to have unlocked the American Championship in Extreme Series, if you have then go there and go on the Indianapolis track and race. You get about 80,000 credits in under 5 minutes which works out to be 960,000 in under an hour. And you get good experience.