If the GTO wasn’t the first muscle car, then what was?

You know the name of first muscle car? It was the Pontiac GTO, of course. Right? Depends on who you ask.

Consider this: If muscle cars are defined as stand-alone American performance cars marketed as such with speed, track times, elapsed times, and other metrics used in advertising, then you’d have to go back almost 10 years before the GTO in order to find the first true muscle car. In 1955, the Chrysler C-300 was the first American car to be marketed by its parent company as a high-performance variant with track numbers to back it up. (The “C” stood for “coupe,” but with subsequent 300s getting a letter designation beginning with the 1956 300-B, the first 300 became known as the 300-A.)

That year, 1955, was a watershed year for the automobile. Car manufacturers were in a cycle of production for almost 10 years after the WWII-mandated moratorium on civilian manufacturing ended. Since that time, Detroit had upped engineering advancements with overhead-valve V-8 engines, reliable automatic transmissions, along with styling that was soon off the charts in terms of flamboyance. By ’55, almost all American cars featured completely new models, with performance becoming a source of advertising hyperbole.

The 1955 Chrysler Corporation offerings were the first year of design director Virgil Exner’s “forward look” styling, combined with all-new chassis and suspensions. Other than the engines, 1955 Chryslers were completely new, shedding their conservative reputation with dynamically engineered and styled cars. The “300” moniker stood for 300 horsepower—an industry first. That 300 hp came by means of two four-barrel carburetors, solid lifters, a hotter cam, and large dual exhaust. A beefed two-speed “Powerflite” automatic was all you could get in ’55, but the following year Chrysler added a three-speed manual transmission. Besides unique checkered flag badges, the distinctive hardtops were based on the lighter Windsor chassis, to which was added the distinctive Imperial front clip, resulting in a unique model for the 1955 Chrysler line.

Chrysler configured the 300s specifically for NASCAR, back when “stock car racing” mandated a stock, production car. The 300s reached a top speed of 127.6 mph in the flying mile—taking the top three wins, and 139.4 mph in 1956, again taking 1-2-3, with the punched-out 354-cubic-inch, 355-hp Hemi. Advertising for the 300 touted wins in both the NASCAR and AAA (later USAC) seasons in 1955. Lee Petty and Tim Flock, both NASCAR Hall of Fame recipients, piloted the Chryslers, with Flock crowned champion that year.

Hudson had a similar NASCAR-winning presence in the early 1950s, but its mainstream advertising barely hinted at that racing dominance. The company never capitalized on Hudson’s dual-carb Hornet successes, and by the mid-1950s it had lost its NASCAR mojo and was soon gone from the automotive landscape.

With just over 1700 built in 1955, the 300’s appeal was limited in part due to the typical Chrysler buyer’s expectations. Featuring a firmer suspension and lopey cam, some 300s were traded in early for a tamer New Yorker or even Imperial. Remember, this was a completely new concept, and no one had ever purchased a new car with this level of “performance.”

Chrysler would continue the letter series Chryslers through 1965, replacing the tall-deck 392 Hemi with the wild-looking and performing 413-cu-in cross-ram wedge in 1959, before slowly taming the handling and performance. By 1962 you could buy a 300 without the hard edges, and without the letter. Chrysler 300s endured into the 1970s, with the limited-edition 1970 300-H (the “H” standing for specialty maker “Hurst”) reviving the spirit of earlier letter 300s. Slightly more than 500 were built, featuring 440-cu-in TNT engines, special fiberglass hoods, trunk lids, and stripes, with an Imperial interior.

The 300 moniker was revived in the 1990s, and the V-6 FWD sedans picked up where Chrysler left off in 1965, with the 300M from 1998–2004, and then with the LX-platform sedans available today. Developed during Mercedes-Benz’s ownership, these later 300s share certain suspension, transmission, and other components with both S-Class and E-Class Mercedes.

Letter-series Chryslers have always been collectible as far back as the 1960s and harken back to the days when Detroit used the lightest sedans and coupes stuffed with the highest horsepower engines available, to carry the performance flag for their respective divisions. As passenger cars continue to take a back seat to SUVs, crossovers, and trucks, Chrysler’s fabled 300 may be one of the last American sedans manufactured.