Classic cars have a reputation for being a lucrative, if not the most lucrative “luxury investment,” but last Monday night they scraped up against some wider realities.

Affluent aficionados of 1960s Ferraris and 1980s Porsches filed past about 50 chanting and whistling protesters on the way to the first London sale of the newly partnered classic car auctioneers, RM Sotheby’s, in Battersea Park. The small demonstration was part of a campaign to reinstate two cleaners who were suspended by Sotheby’s after they had participated in a protest over salaries and sick pay before its contemporary art auction on Bond Street on July 1.

The 1,000 or so dealers, collectors, speculators and spectators who arrived on Monday concentrated on finding out whether classic cars were still a high-performance investment, despite a gloomy economy at their doorsteps and internationally, particularly in China.

It turned out that, for the most part, the engine of growth has slowed. The 76-lot event raised 16.7 million, about $25.7 million, with most lots selling around the low estimate and 26 percent of the cars unsold.

The total was well below the 22 million that RM Auctions of Canada achieved at its equivalent sale in London in September. (On Feb. 18, Sotheby’s paid $30.7 million for a 25 percent stake in RM Auctions, with options to increase its share.) The unsold rate could have been higher, but some sellers lowered their reserve prices before the sale began, or hadn’t demanded a reserve at all. Among 23 lots of the latter was a 1987 AC Cobra Mk IV convertible, which sold to a buyer in the room for just 61,600 with fees, against a low estimate of 120,000.

“People are coming down to earth, even though some sellers still have their heads in the clouds,” said Simon Kidston, a dealer in Geneva who founded the classic car index k500.com last December. “Buyers are financially aware and they get spooked if there is bad news elsewhere.”

In recent years, the top level of the classic car market has been dominated by signature Ferraris from the 1950s and ’60s. A 1958 example of Ferrari’s highly successful 250 GT Berlinetta Competizione “Tour de France” coupe, with a modest racing history and restored original 263 b.h.p. engine, duly topped the sale with a price of 4.8 million against a low estimate of 4.5 million.

In 2013, the HAGI (Historic Auto Group International) Top Index of auction and dealer sales of exceptional collectable cars gained 46.75 percent; in 2014, it was up 15.8 percent; so far this year (up to the end of August), it has gained 8 percent.

“It’s definitely slowing down,” said Dietrich Hatlapa, the founder of the HAGI Index, who, like many observers of the classic car market, attribute much of its top-end growth in recent years to wealthy Americans and Europeans benefiting from central banks’ quantitative easing and low interest rates. “People felt comfortable about having 10 percent of their wealth in classic cars, saw values rise and then increased the capital allocation to 15 or 20 percent. The global liquidity just isn’t there any more.”

This slowdown was also evident in the overall results at the bellwether specialist car auctions in Monterey, Calif., last month. While RM Sotheby’s led the series with $167.2 million of auction sales, including a week-topping $17.6 million for a 1964 Ferrari 250 LM Coupe, the final cumulative total of $396.8 million at auctions by RM Sotheby’s, Gooding & Co., Bonhams, Mecum, Russo & Steele and JR was 7.9 percent lower than the $428.1 million taken at the equivalent public sales in August 2014, according to data provided by Hagerty, a Michigan-based company that values and insures classic cars.

Auctions of classic cars have a singular atmosphere. Crowded with middle-aged men, all of whom seem to know the exact history, condition and value of every lot, these events increasingly only have the capacity to surprise when an extreme rarity comes under the hammer.

“The dynamics of the market are changing,” Mr. Kidston said. “If something is truly exceptional, buyers will pay a good price. If it isn’t, they’ll wait and know another one will come along.”

On Sept. 5, for example, Bonhams achieved a top-estimate 3.3 million euros, about $3.7 million, for a unique surviving prototype of Maserati’s 1972 futuristic Giorgetto Giugiaro-designed Boomerang coupe. It was the stand-out lot at the company’s inaugural auction, which coincided with the Arts & Elegance concours event in Chantilly, France.

On the same day, Bonhams also sold a 1929 Bentley 4.5-liter Sports Saloon in its annual sale of collectors’ cars at the Beaulieu National Motor Museum in Hampshire in England. Remarkably, the Bentley, one of 657 made, had been owned by the same English family since 1935 and was in original “barn find” condition, with its engine seized, having been parked in a garage since 1985. It was bought by a British collector for 695,900, more than four times the low estimate.

“The value is what this is, not what it could be,” said James Knight, group director of Bonhams’s motoring department, who estimated that the Bentley could cost as much as 250,000 to restore. He added that the resultant loss of its once-in-a-lifetime originality could result in a lowering of value. “If it were put back in an auction in restored condition, we’d probably estimate it at 500,000-600,000.”

The auction price exemplified the current vogue for cars in “first paint, first trim” condition, that can compete (if the engine works) in concours beauty parades.

An even more extreme example is the 1928 4.5-liter Bentley that the dealer William Medcalf discovered 18 months ago scattered in pieces around a house at Kew in West London. The only known survivor of eight such Bentleys made with Victor Broom “Drophead” bodywork, it still had virtually all of its original components, including a working engine with its original Bentley “five-year warranty” lead seals in place and a speedometer with 39,000 miles.

“I’ve never seen a Bentley with its original seals,” said Mr. Medcalf, whose Sussex business specializes in restoring and selling vintage examples of the brand. The “Drophead” has now been reassembled to concours condition. “It’s driveable. You can go to Edinburgh in it,” added Mr. Medcalf, who is quoting an asking price of 1.2 million.

As growth slows in the classic car market, has Sotheby’s, who last held auto auctions in 1999, rejoined the party a little too late? Is this really a sector where it can make up ground on its bigger-grossing rival, Christie’s, who stopped selling cars in 2006?

“We are actively pursuing cross-selling opportunities between the two organizations,” Tad Smith, president and chief executive of Sotheby’s, said in the company’s first-half earnings conference call on Aug. 7. “We like the collectible car business.”

In other words, wealthy people buy cars, and being wealthy, they might just want to buy other luxury items that Sotheby’s also happen to sell, like art, jewels and wine.

Moreover, classic cars — unlike art, jewels and wine — isn’t a luxury investment that has attracted much new money from Asia and Russia, and the volatility of demand that goes with it. It’s a business based on American and European men of a certain age and income level who are passionate about buying and driving old cars, albeit rather fewer when economies stall.

“Things can’t keep going up for ever, but I have done well out of my collection,” said Julian Smith, 50, a British businessman based in the Channel Islands who was bidding, but not buying, at RM Sotheby’s auction in Battersea.

Mr. Smith owns, among other cars, a 1973 Ferrari Daytona and an Alfa Romeo 1900C Pininfarina, which he raced in the Mille Miglia competition in Italy this year. “I was more inclined to add to the collection, as all previous cars had appreciated. However, this was not my prime motivation,” he said as he set off into the night, past the Sotheby’s protest. “I just like them.”