Sleepers: The car world's biggest wolves in sheep’s clothing

It would still look like a fast car even if it was powered by a lawnmower engine. On the other end of the spectrum, there is a whole group of cars that conceal powerful drivetrains under the sheet metal of a normal car, like this lovely Ford Mustang V8-powered Rover 75 pictured. From high-horsepower MPVs to track-ready SUVs, we’re taking a look at some of the ultimate sleepers ever sold. We'll also tell you about guide prices for used examples today, and how many we think are left on the roads too:

Pitched squarely against the MGB, the pokiest Sunbeam Alpine engine was a 99bhp unit that displaced 1.7 litres. Keen to make the Alpine more appealing to US buyers Carroll Shelby was asked to pep things up a bit; his solution was to swap the four-pot for a 4.2-litre Ford V8. The result was the Sunbeam Tiger, more than 7000 of which were built between 1964 and 1967.

We found: 1966 Sunbeam Tiger, 68,000 miles - £54,000

How many left?: Around 300

The daddy of them all, created by maverick Merc engineer Erich Waxenberger who shoehorned a 6.3-litre V8 from a 600 saloon into the nose of a 300 SEL. The result was a luxury limo that could crack 140mph and 0-60mph in just 7.3 seconds – deeply impressive figures for a car the size of a (not-so) small bungalow. Impressively, 6525 were made between 1968 and 1972.

We found: 1972 Mercedes 300 SEL 6.3, 55,000 miles - £84,000

Launched soon after the Morris Marina, the Dolomite Sprint was a compact executive car designed to play BMW at its own game. The first production car with four valves per cylinder, the 127bhp 2.0-litre Sprint looked little different from a lowly Dolomite 1300 which had less than half the power.

We found: 1980 Triumph Dolomite Sprint, 67,000 miles - £20,000

How many left?: Around 380

Mercedes-Benz planned to introduce the 6.3’s successor at the 1973 Geneva auto show but it delayed the launch to 1975 due to the oil crisis. Most buyers said the wait was well worth it. The 450 SEL 6.9 was based on a long-wheelbase W116 chassis and it received a bigger, 6.8-litre evolution of its predecessor’s V8 engine tuned to put 286bhp under the driver’s right foot. Period brochures quoted a 0-60mph time of 7.4sec - impressive considering it weighed 1985kg - and a 140mph top speed.

Large and in charge, the 450 SEL 6.9 coddled its occupants thanks to a hydropneumatic suspension system similar on paper to the setup Citroën fitted to its cars. And, here again, Mercedes chose to keep the model as low-key as possible; only a 6.9 emblem on the back set it apart from the 450 SEL. Extremely expensive, about 7380 examples were made between 1975 and 1980.

We found: 1979 Mercedes-Benz 450 SEL 6.9, 77,000 miles - £23,000

Many people assume this was the first ‘sleeper’, even though it wasn’t. But it did create a template for the super-quick mid-size family saloon that married relative affordability with everyday usability. Just look at it – you’d never think that the E28 featured the same 286bhp straight-six as the mid-engined M1 supercar, would you? And unlike later M5s, this first one was little changed from lesser models visually.

We found: 1987 BMW E28 M5, 172,000 miles - £50,000 (listed for sale in South Africa)

The problem with the regular Cossie was that massive rear wing which gave the game away, but the four-door saloon that came later was far more discreet. Yet it still packed the same turbocharged 2.0-litre four-cylinder that gave up to 217bhp in four-wheel drive form – enough to give 0-60mph in 6.6 seconds, yet it looked hardly any more menacing than Gary’s 1.6L rep special.

We found: 1989 Ford Sierra Sapphire RS Cosworth, 46,000 miles - £26,000

How many left?: Around 1000

This is here simply because it’s an anonymous-looking three-box saloon with a Ferrari V8 stuffed in the nose. As such it’s a bit mad, but it’s not actually significantly faster than a Thema Turbo, with its boosted 2.0-litre four-pot engine. While the 8.32 could do 0-60mph in 6.8 seconds and topped out at 149mph, the Turbo – which cost only half as much – was pegged at 7.6 seconds and 140mph. Even worse, the later catalysed 8.32s could manage only 7.2 seconds and 140mph.

We found: 1992 Lancia Thema 8.32, 78,000 miles - £20,000

How many left?: Around 5

It took a well-trained eye to tell the Mercedes-Benz 500E (W124) apart from its cheaper, less powerful siblings. It received a powertrain-specific front bumper, flared wheel arches and, of course, a 500E (later E500 when Merc changed its naming system) emblem on its rear end. Enthusiasts knew what they were looking at but everyday motorists assumed it was an average, taxi-spec Mercedes with a blown-out suspension.

Pushing the front wheels out was necessary to fit a 5.0-litre V8 borrowed from the SL. And, because the 500E was wider than the standard W124, it didn’t fit on the Mercedes production line so it was assembled by Porsche, rather slowly: each 500E took 18 days. The V8 sent 322bhp to the rear wheels, giving the 500E a 0-60mph time of 6.1sec. Mercedes made the 500E between 1990 and 1995 and it sent most of the production run (1528 units) to America. Always slightly in the shadow of the BMW M5, the 500E remains one of the most interesting Mercedes’ in the modern-era.

We found: 1991 Mercedes 500E - £27,500

You were probably expecting the Lotus Carlton in this spot, but that 177mph super-saloon isn’t stealthy enough to be here thanks to its lairy body kit. Much more low key is the Carlton 3000 GSi 24v, which looked little different from cooking versions of Vauxhall’s executive saloon yet it could manage 149mph.

We found: 1992 Vauxhall Carlton 3000 GSi 24v, 70,000 miles - £5750

How many left?: Around 30

Volvo showed its wild side when it created the 850 T5-R. Drawing lessons from its British Touring Car Championship (BTCC) program, and paying for Porsche’s input when needed, it increased the turbocharged five-cylinder engine’s output to 237bhp when a temporary overboost function kicked in. Period road tests suggest it took about 7.0sec to sprint from 0-60mph and it kept going until 155mph.

It was impossible to keep a low profile in a yellow 850 T5-R (pictured) but Volvo also offered the model in black and dark green. Buyers who chose either gave more than a few unsuspecting red-light racers a run for their money. About 5500 examples (most painted black) were made for global markets.

We found: 1996 Volvo 850 T5-R, 53,000 miles - £13,000

How many left?: Around 100

For decades, Volkswagen executives argued they needed to position the Passat as a comfortable, family-friendly model rather than as a hot rod. They notably shot down engineering’s request to make a GTI model in 1977. Officials changed their mind in 2001 when, under the leadership of Ferdinand Piëch (1937-2019), they dropped a 4.0-litre, 266bhp W8 engine into the car’s nose.

The humble Passat now took 6.3sec to reach 60mph from a stop, a time more commonly associated with low-slung sports cars, and it benefited from Volkswagen’s 4Motion all-wheel drive system. Better yet, in many markets it came standard with a six-speed manual transmission. Model-specific wheels, a small emblem on the back, and the muted roar of eight cylinders from quad exhaust tips set it apart from less powerful variants. Volkswagen made the Passat W8 from 2001 to 2004.

We found: 2003 Volkswagen Passat W8, 58,000 miles - £6500

How many left?: Around 70

The Rover V8 is one of the most admired engines of the last half-century, but towards the end of the company’s life it was rather better known for churning out affordable family cars with four-cylinder powerplants. But a handful of 75s (and MG ZTs) got V8 power, using Ford’s quad-cam 4.6-litre V8 (rather than Rover's famous Buick-derived V8) to great effect. Only a handsome V8-specific grille and alloys stood the model apart from lesser 75s.

In naturally aspirated form it delivered 256bhp; a 380bhp supercharged edition was developed for the ZT, but it didn’t reach production. By the way, the 75 V8 Tourer estate version (pictured) is not only a total sleeper, but also a unicorn – just 17 were ever made, and just nine of them are left it seems. Rarity has greatly boosted values in recent years.

We found: 2008 Rover 75 V8, 20 miles - £40,000

How many left?: Around 50

Subaru has a huge back catalogue of models that can seemingly defy the laws of physics. Some are more discreet than others, but top of the stealth pile must surely be the Legacy 3.0R Spec B. No more powerful than a regular auto-only 3.0R, the Spec B got a manual gearbox with ratios designed for better acceleration; it could dispatch the 0-60mph dash in just 6.5 seconds.

We found: 2006 Subaru Legacy 3.0R Spec B, 76,000 miles - £4300

The Phaeton looked as innocuous as a Passat, a car usually sold with a four-cylinder diesel engine. But while the biggest engine you could get in the Passat featured just six cylinders (aside from a handful of W8s), the Phaeton could be bought with a monstrous 444bhp 6.0-litre 12-cylinder engine. If you craved relative economy you could instead choose a 5.0-litre V10 diesel with a ludicrous 553lb ft of torque.

How many left?: Around 20

Audi’s RS models are fearsomely fast but they also feature pumped-up bodywork that gives the game away. Not the S models though; all that gives these away is a set of four exhaust pipes and a very discreet badge. You’d never guess there’s a 435bhp V10 doing its thing up front. Prices now start at £7k by the way...

We found: 2006 Audi S6 saloon, 96,000 miles - £9000

Few luxury cars look more innocuous than the S80. Most of these big saloons were powered by a 2.0-litre diesel engine but it also was available with a silky-smooth 310bhp Yamaha V8 that sent its power to all four wheels. The thing is, the cars with V8 power look no different from those with much humbler engines, with only small V8 badging giving the game away. This is another sleeper that’s also a unicorn – just 100 or so are on UK roads today.

We found: 2009 Volvo S80 V8, 60,000 miles - £12,000

How many left?: Around 100

We’ve already addressed the W8-engined Passat B5.5, but few people remember that despite its complete lack of success it had a successor – the R36 that was based on the Passat B6. Fitted with a 296bhp 3.6-litre VR6 engine the R36 could do 0-62mph in just 5.6 seconds – almost a second quicker than the W8. And the differences from a normal Passat wagon are subtle.

We found: 2008 Volkswagen Passat R36, 68,000 miles - £15,000

How many left?: Around 130

We’ve generally avoided modified cars here because where do you draw the line? However, we have to make an exception for this; a humdrum-looking saloon that packed an 800bhp 6.2-litre twin-turbo V12 that’ll go all the way up to 230mph. That’s faster than any production Ferrari, Lamborghini, Porsche, McLaren or Aston Martin, yet it’ll carry a family and their luggage in comfort – at insane speeds.

It may have looked sharp, but ultimately the Infiniti Q50 was still pretty much invisible. However, those who opted for the hybrid edition had a true wolf in sheep’s clothing thanks to a 302bhp 3.5-litre V6 backed up by an electric motor to give a peak power output of 359bhp. That was enough to give 0-62mph in just 5.1 seconds, so it was a shame the Q50 was so inert to drive.

We found: 2018 Infiniti Q50 hybrid, 58,000 miles - £19,400

How many left?: Around 250

Thanks to Tesla’s forward-thinking design the Model S looks high-tech and desirable, but let’s face it, this doesn’t look like a car that can get from a standing start to 62mph in as little as 2.3 seconds, does it? That’s comfortably (or more accurately, uncomfortably) faster than a Ferrari 488 GTB or Lamborghini Huracan Performante.

We found: 2017 Tesla Model S P100D, 43,000 miles - £56,000

How many left?: Around 450

Spot a debadged M550d and the only clue would be the 19-inch alloys – this was a true stealth machine. Up front was a quad-turbo 3.0-litre straight-six diesel engine rated at 395bhp and 561lb ft of torque, capable of getting the BMW from 0-62mph in just 4.4 seconds – the same time as the previous-generation M5.

We found: 2017 BMW M550d Touring, 63,000 miles - £31,000 (listed for sale in Germany)

So that’s the European market cars – how about the biggest sleepers ever sold in the US?

Although America’s greatest muscle cars were built during the 1960s, Oldsmobile boldly experimented with the idea of putting a big engine in a small car when it released the 88 for 1949. On paper, the recipe was simple: it put the 135bhp Rocket V8 it designed for the 98 into the smaller, lighter 76 body. Its 12.2sec 0-60mph time sounds slow in 2020 but it was extremely impressive at the time.

Performance sold well so Oldsmobile started setting stock car racing records to advertise its new model. It also won five NASCAR Grand National races in 1949. Part of its appeal was that it didn’t look quick; it was offered in six body styles (including a two-door coupe) and they all flew right under the radar.

Chrysler set out to steal the spotlight from Oldsmobile when it introduced the C-300 for the 1955 model year. Its Hemi V8 relied on a pair of four-barrel carburetors and a full race camshaft to deliver 300bhp, a number that made the C-300 the most powerful production car in America when went on sale.

Enthusiasts could only order the C-300 as a coupe priced at $4109 (about $40,000 (£33,000) in 2020). It was Chrysler’s second-most-expensive car, and it looked the part, but only a handful of checkered flag-shaped emblems informed other motorists about the serious power hiding behind the grille.

California-based Crown Manufacturing made some of the more unusual sleeper cars of the 1960s. Starting with an air-cooled Volkswagen, it removed the flat-four and replaced it with a flat-six pulled out of an unsuspecting Corvair. The result, according to period ads, was a 200% increase in power. Speed junkies got a stock-looking Bus or Beetle that could pop a wheelie while spinning its rear wheels.

Production figures are lost to history. We’ve seen only one in the wild in recent years.

In the 1960s, enthusiasts expected automakers to give performance cars a suitably loud look with wings, stripes and vents. Pontiac bucked that trend when it introduced the Tempest’s GTO package for the 1964 model year. Hood scoops, emblems and wider wheels were all that set it apart aesthetically. Its subtlety is forgotten in 2020 because so many examples ended up modified with aftermarket parts.

Pontiac pegged its 0-60mph time at 7.7sec thanks to a 325bhp, 6.4-litre V8 but it was a lot quicker than that in reality. Car & Driver recorded a 4.6sec time when it tested the model in 1964.

There was nothing seriously exciting about the Dodge Omni, a small hatchback that traced its roots to Chrysler’s European division where Brits got it as the Talbot Horizon, yet Shelby managed to turn it into one of America’s all-time greatest hot hatches. The turbocharged, 2.2-litre four-cylinder was tuned to 175bhp, enough for a 7.0sec sprint to 60mph, and suspension modifications greatly improved its handling.

It was a formidable performance car, it managed to out-GTI the Volkswagen GTI, and it lived up to its name: Goes Like Hell S’more. Shelby made 500 units of the GLHS so it’s even rarer than the GLH it’s based on. It’s still an Omni, though, and, few will suspect it’s capable of keeping up with a modern-day Fiat 500 Abarth.

What in the world is a tall, boxy minivan doing here? Ask the folks at Dodge who decided, for the 1989 and 1990 model years, to offer the Caravan with a 2.5-litre four-cylinder turbocharged to 150bhp and bolted to a five-speed manual transmission. Its 9sec 0-60mph time didn’t quite land it in sports car territory but it looked about 10sec slower. Plymouth made the same drivetrain available in the Voyager.

The original Ford Taurus introduced for the 1986 model year quickly became a common sight on America’s driveways, freeways and parking lots. Its ubiquity helped the Super High Output (SHO) model launched for 1989 conceal its performance credentials. The understated body kit signalled the presence of a Yamaha-built 3.0-litre V6 that shifted through a five-speed manual transmission. Putting 217bhp between the front wheels of a Taurus gave it a 6.6sec 0-60mph time.

Ford launched the second-generation SHO for the 1992 model year. The nameplate lived on for years but it retired in 2019 when Ford ended Taurus production once and for all.

Against all reasonable odds, GMC turned the S-15 into a sports car-taming pickup truck by dropping a supercharged, 4.3-litre V6 into the engine bay and tuning it to 276bhp. Its 0-60mph time of 4.3sec allowed it to comfortably outsprint a Chevrolet Corvette while its $25,950 base price (about $49,000 (£41,000) in 2020) made it nearly as expensive as one. It didn’t take long for GMC to sell the 3000 units it planned to make and the Syclone was so popular that its powertrain ended up in an SUV named Typhoon in 1992.

The run-of-the-mill variants of the third-generation Nissan Sentra didn’t exactly set the car world on fire. They were simple, value-packed economy cars designed for thrifty commuters. The SE-R model was the exception to the rule because it received a 138bhp, 2.0-litre four-cylinder engine with a 7500rpm redline. It was front-wheel drive, though Nissan added a limited-slip differential, and it weighed less than 1136kg.

Stylists settled for adding a new-look front bumper with air vents, a spoiler on the trunk lid and alloy wheels. To the untrained eye, it merely looked like a souped-up Sentra.

Often ordered with racing stripes and a hood scoop, the original Plymouth Duster hardly qualified for the “wolf in sheep’s clothing” label. Plymouth resurrected the name as a trim package in 1979, 1985 and 1992; it’s the latter model that earns a spot on this bench. The company started with the Sundance, a mediocre economy car by most accounts, and gave it a 3.0-litre V6 rated at 139bhp.

That was on par with Shelby’s version of the Sundance’s Dodge-badged twin, the CSX, but without the attention-grabbing visual add-ons. The Duster treatment was relatively subtle. The six sent the Duster from 0-60mph in 8.3sec, an admirable time for an economy car.

Chevrolet put the SS designation on some less-than-quick cars over the years so enthusiasts were skeptical when the Impala received it for the first time in decades in 1994. It looked a lot like a blacked-out police car but its 256bhp, 5.7-litre V8 sent it from 0-60mph in 7.0sec. If that doesn’t sound like much, keep in mind it used bulky, truck-like body-on-frame construction and it weighed over 1818kg.

American tuner Callaway offered its own version of the Impala SS equipped with a Corvette-sourced, 395bhp V8 plus a long list of chassis modifications. The extra horsepower lowered its 0-60mph time to 5.9sec, which was about on par with an E34-generation BMW M5. Chevrolet made nearly 70,000 examples of the Impala SS from the 1994 to 1996 model years.

Buick made a serious attempt at luring enthusiasts into showrooms when it introduced a GS variant of the fourth-generation Regal in 1997. Advertised as “the official car of the supercharged family,” it received a 3.8-litre V6 supercharged to send 237bhp to the front wheels. Buick promised a 6.9sec 0-60mph time and a 14.9sec quarter-mile time in a package that looked like a grandpa’s Regal.

The third-generation Mercury Marauder was based on the body-on-frame Grand Marquis and that alone was enough to dispel any suspicions of performance. How can retired America’s chariot of choice appeal to enthusiasts? With a tighter suspension and a Ford Mustang-sourced, 298bhp V8, it turns out.

Mercury redesigned the bumpers, tinted the lights and added 18in wheels to achieve a subtle but purposeful look. It sold 11,052 units of the Marauder during the 2003 and 2004 model years.

The burly Grand Cherokee is part of the Jeep family so it’s normally not associated with any kind of on-road performance. We’d hate to take the Trackhawk off the road, however. It’s more at home on a drag strip because it’s powered by a supercharged, 6.2-litre Hemi V8 engine also found in the mighty Dodge Challenger Hellcat. It makes 697bhp and it spins the four wheels.

The visual differences between the Trackhawk and the regular Grand Cherokee are minor so drivers won’t attract unwanted attention on the road. The exhaust note is, however, a different story…