Best car and place to get this is with the Bugatti Veyron at Le Mans Circuit de la Sarthe Old Mulsanne Circuit I got up to 270mph before braking the achievement will unlock once you finish then quit out of the race.
As noted by BobalobTD and others, Bugatti Veyron at Le Mans Old Mulsanne is the quickest route to this. Please note - if you run the free play default of 3 laps, you must break 250 on your last lap. If you hit 250 on lap 1 but just 240 on lap 3, at end of race it will not pop the achievement.