I think any car that someone wouldn't think is fast but is counts as a sleeper. For example, not many people know of T-Type's sure people here do but as far as general public, not really, they barely know what a Grand National is, most think its a Monte Carlo anyway. So take a built up 10 sec T-Type and joe blow in his bone stock 2005 Mustang GT pull up, "ahh thats just a slow Monte Carlo" The Mustang proceeds to get smoked by the "sleeper" T-Type. Thats my definition of a sleeper.