Like Cob said, you need acceleration control. Remember that the buttons are pressure-sensative. You need to tap the brake to slow down enough to enter the first turn, but after that you really don't need to brake at all if you do it right. Don't push the accelerator all the way down. Just go as fast as you can staying on the inside of each turn, then let off as you get to the end of the turn and get a good angle going into the next one. In the last few turns, you'll barely even be moving. I took the turns at about 100 MPH, then 85, 75, 60, 50, and the last turn at 30 MPH, and that was good enough for gold.
If you're no good at accelerator control, you can just tap the accelerator like the demo car does, but that makes the whole test much less smooth, and it's harder.
