In my previous save, my highest was 286mph with the Chaparral 2J with no slipstream...may have had some tuning, can't remember (increased the top speed to it's max...may go higher with a bit of tweaking); my current save is a low 218mph in my Ford GT05, but then I haven't got past IB yet in this save so that will increase with time
Originally it was the stock Veyron with 420km/h (a bit of slipstream), now that a friend of mine was playing around with my game, he set up a new top speed with a Supra at 508km/h. Ridiculous. Tires would need to be made out of solid steel to resist that speed, and I highly doubt the mass of this car could break through the wind like this so easily. It "barely" reaches 937HP and it oddly feels like it was made for that kind of power, accelerating faster than the Veyron with very little wheelspin and pushing easily beyond 500km/h without N02. I'm not saying the Supra is a bad car, but it's kind of stupid that it can do this in the game.