This achievement is unlocked when you reach 125mph in a car.
Select chapter 11 and play through until you reach the point where you have to drive Leo to the train station. When you leave his house, get into the Ascot Bailey (the sports car) and drive to a body shop and get the sports tuning. Then go to the long bridge to the left of the map and drive full speed down it, use your horn if there are cars in the way and you should hit 125mph by the end of the road.