If you are looking for experience at the expense of some grinding for high endurance levels, race your Formula GT car (assuming you have one) in the Indianapolis race of the Dream Car Championship, it takes under 7 minutes and you get 9819 xp points and 108000 credits which works out to be 78552 xp points and 864000 credits in about one hour of real time.
The Formula GT car shows up in the Used Car Dealership (UCD), but its availability is random--like all used cars. The cars for sale at the UCD are "refreshed" every one day (game time, not real time). With every "day" that passes, six new vehicles show up in the UCD. A day passes when you complete a race or finish a license test. However, you can go through days quickest by loading up a license test and skipping to the next test, without ever actually racing the test. As long as the load screen shows up between tests, one day has passed. It takes five game days to completely refresh the Used Car Dealership's current stock, giving you another chance to find the Formula GT car.
Easter Egg: The Most Realistic ... Die-Cast Models?
The logo of die-cast model car maker AutoArt can be found on the bottom of Nismo Skyline GTR if you flip it -- and possibly other cars. There are also giant screws in the bottom of the vehicle as if the car was modeled after a toy...
For this Trophy you need to "Flip a car and total it." This can be achieved by taking the final chicane on the Trial Mountain circuit almost flat-out in a McLaren F1. Use A LOT of the inside and outside curbs. You probably need to do this twice in one race to get the Trophy.
At level 21, you unlock a race called "Like the Wind" in the expert hall. You most likely do not have a vehicle that will be able to keep up with the LeMans cars you are pitted against. However, golds in both of these races will earn you a Toyota Minolta 88-CV, which will be useful in many races to come. It's a very tricky event, so your car setup and driving technique are crucial. Follow these steps to take the gold and guarantee your first purebred race car.
1. Buy a Chevrolet Corvette ZR1 '09. This car has the upgrades available that you will need.
2. After step one, you will need around $500,000 to upgrade this vehicle.
3. Go to GT Auto, and buy the Racing Modification for the car. (It should be at the very end of the menu.)
4. Now that your Corvette is light and ready for the tracks, you will need power to keep up with those LM Cars.
5. In the tuning shop, buy every high end modification you can, including the supercharger. Be sure to get every bit of HP possible into your Vette.
Now, it's Race Day! Follow the next steps to ensuring gold.
1. You will need to fine-tune the setup. Go to car settings (the wrench icon) and tune your transmission to a top speed of 242. Set the car's downforce (Body and Aero) to the highest possible number on the front and back.
2. The second race on Indy is the toughest, so get that out of the way first: You will need to slipstream to stay up with the others. At the corners, cut inside to overtake your opponents. After a lap or two, you should be pretty close to first, if you haven't reached it. This is the hard part: you will have to block the fast LM cars from taking your spot, so looking backwards in third person is the best way to drive on the straights.
3. After you finally win this one, complete the other. There isn't much to say, but don't slam cars too hard, for your downforce is high and easy to upset. When you pull ahead, you will probably gain a large lead.