The Highland Green 1968 Ford Mustang GT featured in the film Bullitt made audiences’ hearts race – and 50 years it has had well-heeled car buffs reaching for the cheque books.

After frantic bidding at a Florida auction the iconic speed  machine was knocked down for $US3.74 million ($A5.44 million).

The sale at Mecum Kissimmee marks the most expensive Mustang ever sold, surpassing a 1967 Shelby GT500 Super Snake that sold last year for $US2.2 million ($A3.2 million), the Orlando Sentinel reported.

Owner Sean Kiernan, with sister Kelly Cotton riding shotgun, drove the car across the auction block at Silver Spurs Arena and then addressed a crowd of about 25,000 before the bidding started.

“This car had sold twice in its life, it’s been in my family for 45 years. Each time it has sold, it was $US3,500 ($A5,090),” Kiernan said. “So we’re going to start it off at that price and go from there.”

Bidding surpassed $3 million in the first minute. The top bid went back and forth between someone present and a bidder on the phone for several minutes before the mystery buyer on the phone won, agreeing to pay $3.4 million plus a 10 per cent buyer’s premium.

Kiernan’s father, Robert, bought the car after seeing an ad in a 1974 issue of Road & Track. The New Jersey family used it as a daily commuter until the clutch gave out in 1980.

In the late 1970s, Steve McQueen, who played the title character in Bullitt, tracked down the Kiernans and asked if he could reclaim the muscle car in return for a similar Mustang. His pleas went unanswered.

A noted petrol head, McQueen drove the Ford pony car in the movie’s epic chase scene, ordering that the windows be left down so audiences could see it was him behind the wheel and not a stunt double.

Robert and Sean Kiernan began working on the Mustang in 2001. After Robert Kiernan’s death in 2014, Sean Kiernan finished the car and revealed it to the world.

Following the car’s sale, Sean Kiernan said he doesn’t think a more expensive Mustang will ever be sold.

-with AAP