A sleeper (US English) or Q-car (British English) is a car that has high performance and an innocent looking exterior. Sleeper cars are termed such because their exterior looks little or no different from a regular version of the car, but internally they are modified to perform at higher levels, thus they can catch the unaware "sleeping".
Some cars come like this from the factory ready for those who want performance without drama, whether understatedness suits their chosen image or lifestyle, or because they do not wish to attract undue attention of the police. For instance, many high-performance sedans look hardly different from the lower-powered models in the range; for example, the Audi RS4, Acura TL Type-S, Mitsubishi Lancer Evolution RS, Subaru Impreza WRX STI, Cadillac CTS-V, Volvo S60R, Chrysler 300C SRT8, Mercury Marauder, Volkswagen Passat W8, 1996 Chevrolet Impala SS, Nissan Skyline GT-R, and the Dodge Charger SRT8, among others. Cars with external badging, or other visual elements that give the impression of high performance, are not true sleeper cars.
Other vehicle owners create sleepers by swapping more powerful engines, or other performance modifications like turbochargers, and leave the external appearance exactly the way it came from the factory. Sometimes hints of the car's true nature show if one looks and listens carefully: wider tires, a lower stance, or a different engine tone or exhaust note. Gauges and instrumentation are often kept to a minimum. Some owners go as far as to use weight reduction techniques employed by other performance enthusiasts, such as removing items not fundamental to street racing, such as rear seats, interior trim, spare tire, or even the heater.
In some countries, customized sleeper vehicles (as with other heavily modified street cars) may be considered illegal for road use, because the car's level of performance is higher than intended by the vehicle manufacturer; if the owner has focused only on straight-line performance, the existing braking, steering, tires, and suspension systems may have been rendered inadequate. The emissions control system (such as intake and exhaust restrictions, or the EGR system) is often bypassed or removed entirely in customized sleeper vehicles.
Owners sometimes reduce the evidence that their high-performance car is such by removing characteristic badging and trimmings. Sleeper cars often contain stock body work and wheels found on their less-capable brethren to better blend with other traffic and appear unassuming. Some owners simply like having performance without show, but a more predatory use of the sleeper is in street racing, where it is used to fool an opponent into underestimating a car's performance for the purposes of "hustling". Some have even gone so far as to leave their cars' exteriors banged up and rusting, and sometimes even causing additional rusting with the use of battery acid.
Sometimes sleepers will be cheaper to insure when compared to an equally fast sports car, but some insurance companies may refuse insurance to owners of heavily modified vehicles.