Used Mercedes SL vs BMW 6 Series Convertible vs Porsche 911 Cabriolet

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Used convertibles road test - Mercedes SL vs Porsche 911 vs BMW 6 Series

Fancy taking in this summer’s sunshine in something with grace, pace and space to spare? Well then, we’ve got a treat for you. Three delightful used drop-tops all from the upper end of the market, all leather-lined and dripping with kit, and all promising the perfect blend of driving excitement and comfortable cruising manners.

What’s more, depreciation means you can usually buy cars like this for a fraction of their original price. Take the BMW 6 Series Convertible, for example. This version is still on sale today, almost unchanged, as a brand new model, but with a five-year-old example like the one we’re testing here costing a fraction of that price, you’d be mad not to consider it.

The same goes for the Mercedes-Benz SL. Long has this two-seater been the drop-top of choice for celebrities the world over, and not without reason. The latest shape might be a little less elegant than those that came before, but it’s laden with equipment and has undeniable presence out on the road.

And if you’re after something with more of a sporting bent, there’s always the Porsche 911 Cabriolet. It’s a very different car to either of the other two, but no less practical or usable. And it comes with the promise of Porsche’s legendary chassis know-how, which should make it about as exciting a convertible as it’s possible to get. Time to find out which is best.

What are they like to drive?

Even this entry-level 3.5-litre V6 version of the SL gives spirited performance. From rest, initial pick-up is a little stately, but once the engine is at more than 3500rpm, the SL accelerates with far greater urgency. The engine is impressively quiet, too, and as revs rise it produces a sharp metallic exhaust rasp that’s very pleasing on the ears.

The seven-speed automatic gearbox is a bit old-school, preferring to slur rather than rush through the gears. You can counter this by selecting Sport mode and controlling shifts via the steering-wheel-mounted paddles, but if you’re too quick with downchanges, the gearbox simply refuses to play ball.

The mechanical smoothness of BMW’s twin-turbocharged 3.0-litre engine is all the more impressive given that it runs on diesel. It comes close to matching the Mercedes in that regard, in fact, and it’s much more responsive thanks to the torque it produces from just 1500rpm. It hurls the 640d forward, revving quickly to its 5500rpm red line before the sweet-shifting eight-speed automatic gearbox grabs another gear.

Stick the 911 in fifth gear at 25mph and plant your right foot, and you’ll be doing 166mph before the engine runs into the rev limiter. That gives you some idea of just how strong and flexible the 911’s engine is. Rev it out in the lower gears and the 911 gains serious speed, seriously quickly.

Of course, the 911’s real gift lies in its unstickable grip, beautifully judged suspension, strong brakes and sharp steering, which allow you to charge hard and kiss the apex of every bend.

The 6 Series Convertible is an extremely heavy car and its body flexes significantly whenever it hits any kind of bump. Vibrations rattle through the steering column, too. Even so, tight body control and strong grip mean that it can be hustled along twisty roads pretty hastily.

The SL is impressively eager to dart into corners and displays a composure that would shame many a nippy roadster half its size, and yet it’s a proper smoothie in town and on the motorway.

BMW 6 Series Convertible interior

What are they like inside?

The SL’s interior really looks the part, with a petite, leather-topped gear selector, large bullseye air vents and acres of plush leather. It also has truly supportive seats.

With the roof down and the wind deflector and windows up, you’ll sense the world rushing by, but even at motorway speeds not a hint of breeze enters the cockpit.

In the 911, the centre console between the front seats makes the interior feel quite snug, and the sports seats hold you firmly, so backache can set in quite early. What’s more, when the gear stick is pushed forward into the odd-numbered gears, the air conditioning and radio controls are difficult to get at.

Wind blast is well isolated with the deflector in place, though, and the fabric roof is surprisingly effective at blocking out noise.

Much of the dashboard layout and switchgear in the 6 Series Convertible has been lifted from lesser BMWs, but that’s no bad thing because the quality is so good. Entertainment functions are easily accessible through the logical iDrive rotary controller and are displayed on the central monitor.

The 6 Series Convertible also provides plenty of room for four to travel in comfort, but with the roof down there’s quite a bit of wind buffeting. At least the BMW’s boot is big enough to take a couple of suitcases; with the 911 or SL, you’ll struggle to accommodate more than a couple of squashy overnight bags.

BMW 6 series convertible roof up

What will they cost?

Let’s address the elephant in the room first: purchase price. Or, more specifically, the purchase price of the 911. At more than double the cost of the other cars here, its price is in a different league. Of those two, the 6 Series Convertible just pips the SL to be the cheapest of the three.

The reason for the Porsche’s dramatically inflated price is its vastly higher resale value. That does translate as a used purchase – in other words, as a percentage, the proportion of your outlay that you’ll get back when the time comes to sell on will be greater.

However, in cash terms, that still means you’ll lose less money on either of the other two cars, simply because they haven’t got anywhere near as much value to lose in the first place. Even if the 6 Series Convertible was to lose half of its value in any given period, for example, you’d still lose less hard cash on it than you would on the 911 if it lost a quarter of its value over the course of the same amount of time.

The bad news for the 911 doesn’t stop there, mind you. It also has the highest fuel consumption and the highest tax of the three cars, and servicing isn’t cheap either, although the SL will probably cost you slightly more to keep maintained, as Mercedes dealers don’t offer a fixed-price service option for older cars.

All of which leaves the 911 looking like a particularly expensive option. By contrast, it’s the 6 Series Convertible that works out the cheapest, with the lowest initial purchase price, the best fuel economy, the cheapest tax bills and the least onerous servicing cost. The SL, meanwhile, slots in somewhere between the two, though its high service cost and fuel consumption mean it leans more toward the Porsche than the BMW.

Mercedes SL vs Porsche 911 vs BMW 6 Series - group photo

Verdict

This is a difficult test to judge, simply because the Porsche’s cost – both to buy and to run – skews things so considerably. Indeed, we did wonder whether we should even include it in this test, so vast is the difference. But on practicality and usability, it more than holds its head up against the other two cars, and so exceptional are its performance and handling that it is worthy of consideration – even with its vastly inflated price.

That might mean you have to look at an older 911 for your money than you will a 6 Series Convertible or SL. Or, you’re in the lucky position in which money is no object, then you might be prepared to splash out on the Porsche where you wouldn’t on the other two. Were that the case, you wouldn’t be disappointed. It is an exceptional car, a cut above the Mercedes and the BMW in the way it drives and the way it goes, and a genuine sports car rather than simply a sporty drop-top.

For all that, though, we simply can’t judge it as a more savvy used buy than the other two cars here. It doesn’t better them by the order of magnitude that its price and running costs demand, and while we wouldn’t blame you if you wanted to spend the extra to get the most enthralling car here, neither could we recommend that you did so if all you wanted was a comfy, luxurious two-seater.

By contrast, the 6 Series Convertible is the value proposition here. Next to the 911, its running costs are almost city car-like, with impressively high fuel economy, low tax, and cheap servicing costs. It’s also the cheapest to buy. But cars like this need to be comfortable, and with a ride that never fully calms down and vibrations aplenty, that’s an area in which the BMW struggles to deliver. We can’t argue with the good financial sense the BMW offers, or indeed the fact it’s a truly classy cabrio. But it isn’t the best used buy here.

That honour must fall to the SL. Its metal roof makes it quiet and cosseting with the roof up, and with it down, the lack of buffeting is incredibly refreshing. Superb seats make it comfortable, too, and while it can’t hold a candle to the Porsche in terms of excitement, it’s still entertaining enough for most drivers. Indeed, only a slushy gearbox spoils things slightly, but given the nature of the car, even this flaw is one we can overlook.

1st – Mercedes-Benz SL 350

For : Superb sense of isolation, roof up or down; impressive interior quality; great seats

Against : Outperformed by rivals; gearbox can be found wanting

Verdict : A brilliant long-distance cruiser, a lavish sun-longer and a surprisingly agile roadster – the Mercedes SL does it all

Specifications: Mercedes-Benz SL 350
  • Engine size 3.5-litre petrol
  • List price when new £72,495/$108,742
  • Price today £28,000/$42,000
  • Power 302bhp
  • Torque 273lb ft
  • 0-60mph 6.3sec
  • Top speed 155mph
  • Fuel economy 37.2mpg (Official average)
  • CO2 emissions 176g/km
  • Available from 2012-present

The SL is more agile than ever, and a smart interior and folding metal roof add to the wow factor

2nd – BMW 6 Series Convertible 640d

For : Lowest purchase price and running costs; spacious, classy interior; immense engine

Against : Too much body flex; ride too firm; doesn’t feel special enough

Verdict : Delivers in almost every respect, but the unsettled ride is hard to forgive, and it lacks the sparkle of the other two

Specifications: BMW 6 Series Convertible 640d M Sport
  • Engine size 3.0-litre diesel
  • List price when new £72,630/$108,945
  • Price today £26,000/$39,000
  • Power 309bhp
  • Torque 465lb ft
  • 0-60mph 5.2sec
  • Top speed 155mph
  • Fuel economy 49.6mpg (Official average)
  • CO2 emissions 149g/km
  • Available from 2011-present

The soft-top 6 Series provides open air thrills for four, and this diesel-engined version can return almost 50mpg

3rd – Porsche 911 Cabriolet

For : Incredible performance; sensational handling; tactile controls; roof-down smoothness

Against : Exceedingly costly to buy and run; narrow seats; cluttered switchgear; hopeless rear seats

Verdict : A sensational piece of engineering and by far the best car here to drive – but even so, it’s hard to justify its stratospheric cost.

Specifications: Porsche 911 Cabriolet 3.4 Carrera
  • Engine size 3.4-litre petrol
  • List price when new £79,947/$119,920
  • Price today £58,000/$87,000
  • Power 345bhp
  • Torque 288lb ft
  • 0-60mph 5.1sec
  • Top speed 178mph
  • Fuel economy 30.7mpg (Official average)
  • CO2 emissions 217g/km
  • Available from 2011-present

Sports car performance and dynamics turn every trip in the 911 into an adrenaline-fuelled adventure

Price today is based on a 2012 model with average mileage and full service history, correct at time of writing

(whatcar.com, https://goo.gl/Eya63B)

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