Believe it or not, the McLaren F1 was a sales flop. When it was launched in 1993, the world was recovering from a disastrous recession and even the hyper-wealthy baulked at the idea of buying a car that cost more than £600,000.

Never mind the fact that the F1 was the work of one of the greatest car designers of all time, built by one of the most successful Formula 1 teams ever, was the fastest road car in history and had won the 24 Hours of Le Mans. It simply cost too much. Only 64 road cars found buyers, some of them years after it left the factory.

But times and perceptions change. 25 years down the line, the F1 is widely regarded as one of the greatest cars of all time and the fact so few were made means they rarely come up for sale. McLaren has recently strengthed its support for the F1, as well, and a small band of people who lusted after it as kids have now made their fortune. That lot adds up to a massive spike in values. Barely a decade ago, you could pick up a good F1 for a couple of million. This one, which crosses the auction block at RM Sotheby’s Monterey Sale in August, will set the buyer back more than $20 million.

It is a very special example, though. Chassis F1/018 is one of only two cars upgraded by the factory to LM spec, with a race-grade engine and a high downforce bodykit.

After McLaren scored an unlikely win in the 1995 24 Hour of Le Mans, the firm’s boss Ron Dennis decided to create a run of track-focussed F1 road cars based on the specs of the victorious F1 GTR race car. The F1 LM was fitted with the GTR’s race-spec engine, but freed from the restrictors dictated by racing rules so it was good for 680hp. A full kit of aero devices was added, as well, giving more downforce than the race cars. They did affect the top speed, though, the extra drag limiting the F1 LM to pedestrian 225mph.

Six examples of the F1 LM were built (including the prototype) three of which disappeared into the Sultan of Brunei’s collection. Two standard F1s were subsequently sent back to the factory to be upgraded to LM spec.

Chassis 018 was built in 1994. Its second owner commissioned the upgrades in 2000, based on the LM package but taken a few steps further. In the process, the cooling system and exhaust were beefed up, race-spec suspension and wheels added, and a smaller steering wheel fitted. Whilst they were at it, the aircon was improved, the interior retrimmed, the body repainted in Platinum Silver (it had originally been Midnight Blue) and – luxury of luxuries – a radio was fitted.

It has been through several owners since, all of whom have had it meticulously serviced at the factory. In fact, it’s very highly regarded there, F1 service program manager Harold Dermott saying in a letter to the current owner: “F1/018 is one of my favorite F1s and one of the most heavily developed cars that we have ever built.”

If the man who knows F1s better than anyone else on the planet says that, you know that this is a good one.

The current, New Zealand-based owner certainly enjoys the car, taking part in many F1-only tours around Europe. It’s racked up just over 13,000 miles and is described by RM Sotheby’s as the “best imaginable” example. High praise indeed.

All of which more than justifies the estimated sale price of $21 million to $23 million. If it achieves that, it’ll be an auction record for an F1, possibly for British car and definitely for a car built after the 1960s. It’s also an increase of $10 million on the price paid for the other upgraded-to-LM-spec F1 just four years ago.

Still, chassis F1/018 is actually a bargain. If one the six original F1 LMs ever comes up for sale, it would probably sell for well over $50 million…

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By Graham King