I could go on for hours on this subject so I will do my best to make it brief. My definition of a sleeper is a car you had no clue was quick, looks very unsuspecting or above all rare.
The car must also be quiet and appear to be stock. Usually tires and a roll bar are a dead give away that the car is packin' heat. Other ways to tell are by the stance.
To me there is no such thing as a factory sleeper IF you know your cars. SRT8 Jeeps can be sleepers to the average person, while people like you and me know it'll give us a run for our money if the guy has a dime into aftermarket parts.
A sleeper must not look fast either. To me no Mustang or Camaro is a sleeper unless it's running quicker than 10s and is on the street/in street form. We all know the potential Pony Cars have so if they are quick it should come as no surprize. The only real vehicles that are sleepers to me (or unsuspecting) are Diesel Trucks. They seem to sound the same to me if they are factory or modified. The only thing that can give it away is the smoke stack(s) and even then you don't know what you are racing until they get moving.
I was once told anything that runs 13s or faster is a sleeper, this I do not agree with. There is no time you must meet to be a "sleeper" it's all about appearance. When my K5 was pushing mid 15s a lot of people considered it a sleeper because no K5 was going that fast or at least none were known to do that. I have seen footage of a 14 second Semi truck... who would have thought? Finally on DVD I have a Supra running 9.001 and killing a Motorcycle and the Supra looked completely stock.
Those are sleepers. The worse they look the cooler it is.
What is your definition? Do you have any videos or pictures to share?
Post em up!