For this achievement, you can be driving any car and be in any game mode - Whether it's in free roam or a race, this will unlock.
It is confirmed that you can go to a car showroom and test drive a car and get the achievement. I'd highly recommend using the Bugatti Veyron.
Simply go to the European Car Dealership "European Cars (south)" just north of the Airport and test drive the Bugatti Veyron. When you are outside of the dealership, make sure you're going to the right - Now just floor it and dodge the traffic until you hit the speed.
Alternatively, you can purchase the Bugatti Veyron and go to Hawaii. Once you head out of the Airport - Head toward the right (East) and floor it. This is possibly the longest and straightest piece of road in the whole game, so this road is by far your best bet.