You can see your speed in the top left hand corner. This achievement can be achieved in episode 4 or episode 5(or perhaps even in episode 3 with some dedication). Some cars featured in the daily event episode 4 are able to reach 400kmh+. Simply pick up nitro boosts, make stunts and knock down cars until you have a full pink nitro bar. Then use nitro by pressing the space bar and get the achievement.
Don't worry if you don't get the achievement in episode 4, you''l definitely get it in the higher episodes, where cars are able to reach even higher paces.