This 1967 Shelby GT500 will soon be shown off at Mecum's next auction event, taking place in Kissimmee, Florida during the week of Jan. 3. The custom Mustang was built by Carroll Shelby for an event promoting Goodyear Thunderbolt tires. Given its rarity and unbelievable specs, Mecum expects this unicorn among more common ponies to fetch at least $1 million once it makes its way across the auction block.

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At a glance, the Super Snake looks about the same as any other Shelby Mustang, but a roll cage inside and a massive radiator up front hint that something more is going on. Pop the hood, and you'll see that this car is fitted with what Shelby engineer Fred Goodell called "the mother of all 427s." The 7.0-liter big block V8 was sourced from Ford's GT40 Mk II and features aluminum heads as well as forged internals, including the crankshaft and piston rods. Mecum claims it produces around 600 horsepower, so this is essentially a high-strung Le Mans racing engine dropped right into a 1960s streetcar.

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To promote the Thunderbolt tires, Carroll Shelby fitted the Super Snake with four of them, then had Fred Goodell drive the car 500 miles around Goodyear's oval test track. The Mustang reportedly topped out around 170 miles per hour and averaged 142 during the endurance test. The Thunderbolt tires also proved to be a success, retaining 97 percent of their tread after the run was over.

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The Super Snake was meant to be a prototype for a line of specialized racing GT500s, but the project turned out to be too expensive for production. Since its 500-mile run, the car has been displayed at a Ford dealer in California, had a short career in drag racing, and then changed hands through various collectors. A previous owner returned the Super Snake to as close to its Goodyear test specification as possible, even tracking down a surviving set of Thunderbolt tires to mount on the car.

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