The term muscle car generally describes a mid-size car with a large, poweful engine (typically, although not universally, a V8 engine) and special trim, intended for maximum acceleration on the street or in drag racing competition. It is distinguished from sports cars, which were customarily and coincidentally considered smaller, two-seat cars, or GTs, two-seat or 2+2 cars intended for high-speed touring and possibly road racing. High-performance full-size or compact cars are arguably excluded from this category, as are the breed of compact sports coupes inspired by the Ford Mustang, the "pony car". Another factor used in defining a classic muscle cars is age and country of origin. A classic muscle car is usually but not necessarily made in the US or Australia between 1964 and 1975.
An alternate definition is based on power-to-weight ratio, defining a muscle car as an automobile with (for example) fewer than 12 pounds per rated hp. Such definitions are inexact, thanks to a wide variation in curb weight depending on options and to the questionable nature of the Society of Automotive Engineers (SAE) gross hp ratings in use before 1972, which were often deliberately overstated or underrated for various reasons.
Another alternate definition involves a car's original design intents. Muscle cars are factory produced automobiles that have a larger engine than was originally planned for in the design and production phase of the original car. Examples of this trend can be found throughout American, Japanese, and European cars of all designs. This includes many cars that typically are not labeled as muscle cars, such as the B13 & B15 (1991-2006) Nissan Sentra SE-R & Spec-V, and excludes other cars typically labeled as muscle cars, such as the Dodge Viper.
