BMW 640i Convertible

What is it?

According to designer Nader Faghihzadeh, some of the shapes in the BMW 6 Series are inspired by the passage of water. Imagine poking a stick into a stream and the resultant twin ripples trailing behind it, and you have the gently curving creases emerging behind the 6's nose badge, to arc towards the bonnet's edges before streaming up the 'A' pillars.

The movement of water has added appealingly muscular sculpture to its flanks too, creating a lither, more elegant machine than its predecessor. This all-new 6 rides on the architecture of the latest 5 Series, benefitting from four-wheel steering, a cabin-enlarging wheelbase stretch, new engines and part-aluminium, part composite bodywork, although it still weighs a substantial 1840kg.

Still, the spectacular 50 per cent gain in structural stiffness is fine compensation, as are fuel-saving features such as brake energy recuperation and stop-start, though that's only fitted to the entry-level, turbocharged 3.0 straight-six sampled here.

What's it like?

This engine is 48bhp more powerful than the outgoing 630i and only 12bhp short of the old 645i, its creamy 332lb ft of torque peaking from a diesel-ish 1300 to 4500rpm. It'll rev to 7000rpm, although short-shifting at 5500rpm is speedier.

There's pleasure to be had from paddling too, this big convertible agile enough to play the sportscar even if the electro-mechanical steering veils much of the road below. Confident turn-in, four-wheel steering and a remarkably tremor-free structure all contribute, as does standard-fit dynamic drive to make a fine long-distance cruiser.

While it lacks the 650i's burbling potency it's brisk enough for most, rides better and turns in almost 10mpg more and 64g/km less. Down-sides? The stylish fabric roof makes access to a roomier but still-confined rear, the boot remains small, rear visibility is appalling and the 'A' pillars are apex-obscurers in tight turns. A crude seat-height adjuster removes thigh support as the cushion rises, too, and throttle response from rest can be abrupt.

Should I buy one?

But this is a superior 6 Series convertible, better able to play the dual roles of luxuriously svelte cruiser and agile bend burner, and this six cylinder is the shrewder buy - though waiting on the diesel may be shrewder still.

BMW 640i SE Convertible

Price: £65,680; Top speed: 155mph (limited); 0-62mph: 5.7sec; Economy: 35.8mpg; Co2: 185g/km; Kerb weight: 1840kg; Engine: 6-cyls in-line, 2979cc, petrol; Power: 316bhp at 5800-6000rpm; Torque: 332lb ft 1300-4500rpm; Gearbox: 8-spd automatic