Super easy to get without having to buy, upgrade or tune anything. You should also unlock the following achievement in the progress:
What you need to do is from the main menu go into:
Free Play > Hot Lap > Bugatti > Veyron 16.4 > Le Mans > Old Mulsanne Circuit
The plan is to reach 240MPH (~390KMH) on the extremely long straight. Just drive like you normally would for the first section and when you get on the straight floor it.
It'll take a while to get up to the intended speed but you will eventually reach it, most likely on your first attempt.
If you, against all odds, haven't reached the speed required when at the end of the straight you can always turn around and drive down the straight again!