I recommend driving circles in the Bonneville salt flats. I used a toothpick to jam my thumbstick and a rubber band to keep it accelerating. Now you cannot
idle boost this, you have press a different button or wiggle the other thumbstick every 10 minutes or so.
I recommend using a phone charger / micro usb cable to keep the controller from draining batteries or turning off.
I hit about 360-370 km/h any faster and my car would make too big of a turncircle and eventually crash. To detect this, leave your vibration on. If it vibrates slightly and a little bit you're fine, but if it's more than 3 times in a row or an intense vibration than you'll have to correct or restart circling.
I did this today as Netflix uploaded an entire season so I could watch that on a different channel of my tv.
Also if you've hit the number but didn't get the achievement yet, pull over