Without question, the Pontiac GTO's legacy of being the definitive muscle car will carry on for years despite the brand's demise in 2010.
The Pontiac GTO is the most important of all muscle cars. Which one is best would be a matter of taste, but the GTO was the moment of genesis for the muscle car era, and truthfully it was quite an excellent example of the breed in its own right. The Ford Mustang would come out later that year, launching the pony car niche, and thus began one of the most exciting eras in the history of American cars. The idea of putting a big engine into a cheap midsize car in order to make speed available to the masses might seem like one which required little thought.
But it's one which took a bit of time to really get moving. The 1949 Oldsmobile Rocket 88 is really where the idea began. It was made by taking the advanced (for the time, obviously) OHV high-compression V8 out of the big 98 series cars and putting it into a 78 series body. The car was a big success in NASCAR, the first to so totally dominate the racing series, and this success carried over into big sales figures. But then GM forgot all about it. In fact, just before the GTO was first introduced in 1963 for the 1964 model year, Pontiac's own sales manager, Frank Bridge, was adamant that it would be a flop and that production numbers should be kept low.
The idea came about as a result of a weird rule implemented by GM in 1963 which banned all of its divisions from participating in motorsports. Pontiac's corporate image was closely tied to performance, and this was in danger of becoming irrelevant without any kind of outlet. So it was decided that Pontiac would instead offer something to rule the streets. The newly-redesigned Tempest was chosen for the platform, and the car was made simply enough by just shoehorning in a bigger engine. The name was rather unapologetically lifted from the Ferrari 250 GTO, despite being completely meaningless on the Pontiac.
However, the three letters do have a nice sound to them as well as a certain association with performance. The GTO was actually a violation of another rule at GM that mid-size cars couldn't have engines bigger than 330cu-in. The 383 V8 which produced 325 horsepower (or 348 with the optional "Tri-Power" carburation) in the GTO was pretty blatantly bigger, but Pontiac argued that since it was part of an option package and not a standard engine, that this constituted a loophole. This probably didn't make the bosses happy, but when they received 10,000 orders for the GTO before the 1964 calendar year had even started, they relented.
Final 1964 sales figures totaled 32,450 units, a number which proved that Frank Bridge's initial estimate of 5,000 units to have completely misjudged the market. Competition from both inside and outside GM immediately heated up, even before the end of 1964. But demand increased too, and more and more people realized that they too would like a muscle car. Sales for 1965 were more than double the year before, and would continue to climb. There was also a redesign for 1965, and power was bumped to 335 horsepower for the base model and 360 for the Tri-Power. 1965 saw another redesign, and GM brass now had to admit utter defeat in deference to their 330cu-in rule.
As such, the GTO became a separate model from the Tempest. Sales for the year were the highest ever for the GTO, coming in at 96,946. Sales stayed strong into 1967, which also saw the engine size increase to 400cu-in. Pontiac had been aggressively marketing the GTO as the "GTO Tiger", but by this point, the slang name "The Goat" had pretty effectively taken hold, and Pontiac's marketing efforts proved futile. The body was radically changed for 1968, giving it a much more aggressive look, although powertrain options remained the same. Sales remained strong despite ever-increasing competition from several different Big Three brands.
There followed the introduction of a number of ram air performance upgrades, as well as the "Judge" option package to keep the car competitive. But the GTO was still losing the performance war to the Plymouth Road Runner. So for 1970, Pontiac introduced a massive 455cu-in V8. This was rated at 360 hp and 500lb-ft of torque, but this was the height of under-rating horsepower figures for big engines, and debates are still going about what the engine's true output was. In 1971, GM was preparing for the implementation of unleaded gasoline, and compression ratios were lowered on all of the GTO's engines, thus decreasing power.
Sales had been down since 1970, but they took another huge dive in '71, and the GTO went back to being an option package for the Tempest in '72. A new GTO debuted in 1974 which, though less powerful, was much lighter, and actually put out decent performance numbers. The car sold better than the previous two years, but the public had soured to muscle cars, and the new GTO lasted only one year. Several attempts were made to revive the name, but it wasn't until 2004 that a proper fourth-generation of the car was introduced. This was a rebadged Holden Commodore, and unfortunately it just wasn't quite good enough to stick around.