If you want to draft for fuel efficient speeding, then right behind. If you want to avoid a ticket, 1/2 mile behind. If you are being chased by a mad person, then as far ahead as possible. If you have no brakes and your throttle is stuck, then behind as the front car gently slows you down. If you are having a street race for pink slips, then well ahead. If your wife is pregnant and you're getting a police escort to the hospital, then behind. If you're in California, you're still not going fast enough.