WIKIPEDIA: A sleeper is a car that has high performance and an unassuming exterior. Sleeper cars are termed such because their exterior looks little or no different from a standard or economy-class car. While appearing to be a standard or neglected car, internally they are modified to perform at higher performance levels
This E46 from Last Minute Racing looks like any other non-M 3 series on the road. Take a closer look!
E46 M3 engine, 6speed transmission, brakes, suspension, rear end, interior etc etc etc. The only thing left bone stock was the exterior, and that is why this car is the ULTIMATE sleeper!