Mecum Auctions(KISSIMMEE, Fla.) -- The most famous Mustang in the world is also now the most expensive Mustang ever.

The Highland Green GT fastback that tore up the streets of San Francisco in the 1968 movie "Bullitt" sold for a record $3.4 million at auction Friday in Florida. The final price, which includes a buyer's premium, is $3.74 million. There was no reserve. Mecum Auctions has not named the buyer but a company spokesman said the winning bid was placed over the phone.

The previous record holder for a Mustang was a 1967 Shelby GT500 Super Snake, which was bought for $2.2 million in January 2019.

The 1968 Mustang had belonged to the Kiernan family for 45 years. Sean Kiernan, whose father, Bob, purchased the vehicle for $3,500 in 1974, decided to sell the car after showing it around the globe for the last two years. Car enthusiasts were stunned to learn in 2018 that the largely unrestored Mustang had been hiding out in Kiernan's Kentucky barn for decades. The last time it was driven was in 1980.

McQueen tried to get back his movie co-star in 1977. He wrote a letter to the Kiernans, offering a small sum for the car as well as a replacement.

“Again, I would like to appeal to you to get back my ’68 Mustang. I would like very much to keep it in the family in its original condition as it was used in the film, rather than have it restored; which is simply personal with me,” McQueen said in his letter, dated Dec. 14, 1977.

“I would be happy to try to find you another Mustang similar to the one you have, if there is not too much monies involved in it. Otherwise, we had better forget it,” he concluded. “With kindest regards, I remain very truly yours, Steve McQueen.”

Eric Minoff, a specialist in the motoring department at Bonhams, said the sale was a win for Sean Kiernan even though the car did not fetch $5 million as some had expected.

"The seller should be happy," he told ABC News. "It sold for nearly a thousand times what his father paid for it."

The record sale also attests to McQueen's enduring appeal with car collectors and movie buffs.

"McQueen is still a popular character who holds sway," Minoff said.

Bonhams will be auctioning off two vehicles driven by McQueen in March: a Con-Ferr Meyers Manx Dune Buggy and a 1967 Rolls-Royce Silver Shadow that were featured in "The Thomas Crown Affair."

Another "Bullitt Mustang" may be headed for auction soon. The stunt car beat up in filming was found in Mexico around two years ago. Its current owner is in the process of restoring the vehicle to its former glory and has expressed interest in selling it.

Kiernan told ABC News before the sale that he was ready to say goodbye to his father's car after all these years.

"I am OK with any price. But I would like it to be the most valuable Mustang ever," he said.

Mission accomplished.

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