Nothing happens in the motor industry without a reason, and the reason why Mercedes-Benz launched its new SL in California is that more are sold in the US than in the rest of the world put together. Out here in California, where locals will tell you the sun shines 300 days every year, what better conveyance could there be?

The pace of life is a lounge lizard crawl that suits the SL to perfection. Climb aboard and your arm gravitates to the window sill. On go the shades, down goes the roof and as you select “D” on the gear selector, you engage “cruise” in your head. And if all you did was waft about, marvelling at the cosmetically enhanced culture of this part of the world, you’d find an SL ideally suited.

But there is another kind of SL, one with a steely eye and a sharper focus that would be wasted if used only for such untaxing purposes. It used to be called the SL 55 AMG but now it’s been replaced by this, the SL 63 AMG and, as the name suggests, its engine is not only bigger, it’s more powerful too.

On paper, all appears to be well. Confusingly, its engine displaces 6.2 litres but few are likely to complain about a phantom few cc when it has 525bhp and will push nearly two tonnes of SL to 62mph in 4.6sec. It may lack the low-down shove of the old supercharged 5.5 litre engine but it’s a sight more frugal and less polluting.

Better still, it comes with a transmission new not just to the SL but to any car. In essence it’s a seven-speed automatic gearbox but instead of being mated to the engine via a torque convertor, it is attached mechanically via a clutch. The theory is it will provide instant gear changes in manual mode and smooth automatic shifts when it is not. Mercedes says it will change gear in a tenth of a second, as fast as a Ferrari 599 GTB.

Yet it was not a car I warmed to at first. For a start I think the new nose has wrecked the styling of a car I always considered to be unusually pretty. Second, while the engine delivers on all its performance promises, it needs to be worked hard, which is what you expect and even hope for in a Ferrari, but not a car that remains at heart a long-distance tourer. And even the trick gearbox is not quite what it seems: while it may well take no more than 100 milliseconds for a gear to be swapped, there is a small but distinct pause between your finger making the request on the steering-mounted paddles and the gearbox responding. On balance I’d say a standard DSG-type twin-clutch semi-automatic gearbox, like that available in a humble diesel Golf, is smoother and more satisfying.

But just when you think Mercedes has blown it, the SL 63 shows you another side to its nature that makes you think again. Because of its size, weight and demeanour, this is not a car that begs to be thrown down a difficult road. It seems more of a straight-line squirter than a Broad bruiser. Yet show it a mountain pass and it attacks it with relish. It corners flat and fast, fulfilling the promise of its ugly but aggressive new looks to the letter. And then, once you and your car have had your exercise, you just ease off the gas, select “Comfort” mode for the gearbox and revert to the SL’s long-established type.

Were the AMG less capable than it is, it might appear a confused and even compromised car, unsure whether it is a sprinter or a marathon runner. In fact it’s so good at both, it’s a clearly defined all-rounder and comfortable in the role.

And yet it is missing something. I once drove the SL 55 AMG from London to Geneva in 11 hours and loved it so much I didn’t mind handing over fistfuls of euros to the French police. Though the SL 63 is a better car, the SL 55 had a charm its replacement lacks. Not only was it a trifle quicker than the SL 63, it was effortless in a way all SLs should be – you just twitched a toe and it was gone.

That said, had I never driven an SL 55, I’d now be hailing the SL 63 as the fine and accomplished product it undoubtedly is. As it is, the best I can say is that it’s a worthy successor, worthy – as opposed to thrilling – being the operative word.

Vital statistics

Model Mercedes-Benz SL 63 AMG
Engine type 6208cc, eight cylinders
Power/Torque 525bhp @ 6800rpm / 464 lb ft @ 5200rpm
Transmission Seven-speed automatic
Fuel/CO 20.3mpg / 330g/km
2 Performance 0-62mph: 4.6sec / Top speed: 155mph (limited)
Price £101,975
Road tax band G (£400 a year)
Verdict More capable but less appealing than the SL 55

Date of release April

The opposition

Model BMW M6 convertible £88,225
For Characterful motor, flashing performance
Against Ugly, unremarkable ride, fabric roof

Model Jaguar XKR convertible £76,097
For Pretty, fun to drive, charming cabin, comfortable
Against Slightly insubstantial feel, cramped rear seats