IT'S so similar to the model it replaces, yet vastly different in so many ways.

The last BMW 6-Series Convertible had a huge following for its macho styling in-your-face aggression, not to mention its punchy performance.

The latest version - immediately recognisable and apparently inspired by the ‘flow of water' - is more smoothly contoured, has greater elegance and possesses a subtlety lacking in the old car.

Yes, it is still enormously powerful but, as if to underline the new approach, the 640i is unusually economical for a 155mph grand tourer I drove the convertible version which precedes the coupe.

Now a tad larger, the area that the increased size is most noticeable is the cabin which can now claim to be genuine four-seater. The boot, slightly awkwardly shaped due to the space required for the fold-down fabric hood, holds a modest 350 litres of cargo.

I opted for the less powerful six cylinder 3.0-litre 640i rather than the hairy V8 engine 650i, as I reckon most buyers will who are remotely interested in containing fuel prices.

The difference in performance is marginal - 5.7 seconds to 62mph in the smaller engine car against 5.2 seconds in the monster, yet you can expect almost 10 miles more from every gallon driving the 640i. Both models are electronically restricted to 155mph.

For the record, my average was an impressive 33mpg over the 600 miles I covered in the 640i.

Despite the deep reserves of acceleration, the twin-turbo,316bhp 640i is a bit of a pussycat for normal driving.

The eight speed automatic gearbox, which can be switched to sport or manual modes and used via steering wheel paddles when the urge strikes, is sweet and seamless and the substantial torque means that there's a ratio for every situation.

Throttle response is immediate and the throaty sound emitting from the dual tailpipes reminds you that this is a sports car as well as a grand tourer.

Ride quality is firm enough to all but eliminate roll, yet has sufficient compliance to soak up most bumps and undulations.

There is, however, a flaw in the handling department that detracts somewhat from the 6's driving enjoyment - the steering feels rather remote and fails to communicate all the road messages back to the person at the helm.

Gone are the days when BMW cabins smacked of Teutonic austerity, albeit solidly built and well-screwed together.

The 640's facia and interior as a whole is opulent, elegantly styled and beautifully built. The dash is dominated by a huge central screen which can be sat-nav or onboard computer-cum-control module. It all works well and easily and you don't have to be a techie geek to operate it.

As you would expect in a model costing £65,695, mile-eating journeys are consumed quietly and in total comfort.

Perhaps surprisingly BMW opted for a multi-layer fabric top for the 6, despite using a metal-folding  roof on the cheaper Z4.

One of the reasons given is that the marketing people wanted to easily distinguish between the convertible and coupe versions.

In the States there has been some sales resistance to the steel-top 3-Series convertible because it looks too similar to its coupe counterpart.

Fast, frugal and extremely swish, this a truly grand tourer.