For this achievement, you have to have reached 240 mph in any car. An easy way to get this achievement is that you want to go into an online match, pick the SSC Ultimate Aero and for a track, pick the Old Le Mans track - NOT the Le Sarth one, as there are kinks in the straight away that will not allow you to get up to that speed. Doing it in this car is relatively easy, so you should unlock the achievement in a matter of a few minutes.