To me there are different levels of sleepers. The Wheel 2 Wheel Nova might have some rumble at idle and bigger tires on it, but the car absolutely flys too. That's a car you might look at and wonder if it could run in the 10s. But it's really in the 8's.

My junk couldn't come close to competing with that Nova, but mine idles like it's stock and has 255/60 tires on it too. So yeah, you could look at it and wonder if it might run in the 12's but it should be into the 10's easily.

Loud or quiet, rough or smooth idle, big or little tires… Sleeper simply put just means it runs faster then what you think it should.