This achievement requires a fast sports car, like the De'Leo Stiletto, which can be found (rare) randomly driving around in the Frisco Fields region (north-west of map). Other high-end sports cars also work, like the Samson Duke and Samson Opus, which both can be found (rare) randomly anywhere in the game. The route you want to take goes from south-west Frisco Fields over the bridge to Barclay Mills, through the tunnel and down the hill. Just follow the main road after passing the bridge. Since you will be driving downhill, you can easily keep your speed. Later in the game, you can also unlock upgrades that improve the speed of your vehicle. Thus, if you have any trouble, you can wait until later in the game.