275 mph in a Cadillac Cien with NOS. I ran minimum downforce, and lots of camber (in real life camber hides a bit more of the tire in the wheel well, reducing wind resistance, though I doubt the game included that). I put no toe because there isn't any initial acceleration at those speeds great enough to cause the shafts to twist. Lastly, the car was at near perfecting balance in terms of gearing- a shorter gear woulld have limited the top speed by wind resistance, and a slightly higher gear would make it rev-limited.