BMW 6-Series 640d M Sport Convertible

Overall rating

With its three-litre turbo unit BMW believes it has built on the success of the previous generation oil-burning 6 Series Convertible. It has again created a diesel engine smooth, refined and powerful enough to see duty in a luxury grand tourer without detracting from the quality of the overall driving and ownership experience.

Having already proved its ability to provide pace, economy and refinement in the 5 Series and 3 Series platforms it was only ever a matter of time before BMW's latest three-litre, six-cylinder diesel unit found its way into the 6-Series.

Key to the engine's success is its ability to revs as freely and smoothly, if not as high, as a petrol engine. Its torque, which is delivered in huge quantities and from extremely low in the rev-range, means the 640d has acceleration akin to that of the more powerful petrol-engine 6-Series models. In-gear acceleration is equally impressive.

The 640d is also able to return fuel economy the likes of which rare in the GT sector. In the past, GT ownership inevitably came with the high running costs of petrol powered cars. In comparison, the diesel 6-Series will leave a considerably smaller dent in both the owner's pocket and environment.

Committing the diesel unit to a convertible body, where there is no room to hide from any unwanted noise or roughness, is probably the ultimate show of faith in the technology. Installed in the second generation 6 Series, the overall achievement is very close to that of a tin-op car with the roof up and no discernable negative experiences with the roof down.

Our verdict on the BMW 6-Series 640d M Sport Convertible

The 640d is no less refined or enjoyable to drive than any of BMW's petrol models. Granted, it doesn't have a spine tingling exhaust or engine note like the M6, but that's a small price to pay for the impressive fuel economy and the even more impressive way that it doesn't come at the expense of performance.

Costs

The 640 is probably one of the most frugal GTs you'll find, with the potential to cover more than 600 miles on a single tank of fuel. While running costs are modest by executive level standards, the car is still likely to incur a high insurance premium and aspects like the run-flat tyres will not be cheap to replace.

Space and practicality

As you'd expect, the car's folding roof mechanism impedes on the boot space available in the 640d convertible, but there's still enough room for a reasonable amount of luggage. The 6 Series' forte is not as a practical run-around, but it is spacious inside, particularly for front passengers to play at being an all-rounder if required.

Controls and display

The car's paddle shift Sports automatic transmission works well but lacks the urgency of a conventional manual or even a direct shift (DSG) 'box. Elsewhere, instruments and controls are easy to view and conveniently positioned. As with many BMW models, it takes some time to get used the various column stalks and their functions – but their operation makes sense once you are familiar with them.

Comfort

The skilfully appointed interior of the 640d is as comfortable as you would expect from a premium GT with a price tag to match. Long journeys fail to take their toll thanks to adjustable seats and the absence of wind noise from the cabin. Sculpted seats for the rear passengers offer a degree of comfort.

Car security

A comprehensive alarm system is standard, complete with tilt alarm and a battery back-up. There's plenty storage to conceal valuables inside the car, and the contents of the boot cannot be viewed from outside.

Car safety

As well as a vast array of safety systems to keep the power in check, the car can be had with a lane departure warning system that activates if it detects a possible lapse in driver concentration. Rain sensing wipers and automatic headlights are also standard as are huge brakes. They're intelligent too, with automatic drying, pre-tensioning, fade compensation, soft-stop and hill-start assistant.

Driver appeal

In 640d guise the ample torque of the diesel engine makes it a more pleasurable driving experience than a similarly powerful petrol unit. Acceleration is swift and effortless, and the sports suspension and rear-wheel drive set up makes it an agile handling machine. The car has the ability to fill a driver with confidence but is equally adept at cruising and the daily grind.

Family car appeal

Although more sensible than one of the thirstier petrol variants, the 640d convertible would make a poor choice for those with a family in tow. Passengers are limited to four and luggage space would not be adequate for holidays.

First car appeal

It may be on the wish list, but only a very lucky few will ever find themselves behind the wheel as a first car. A degree of driving experience is required to make full use of the performance and to cope with its bulky size around town.

Quality and image

The looks won't be to everyone's taste but the second-gen model is softer on the eye and sportier in its styling. The build quality is excellent with no rattles, refinement even at speed and a finish of the highest order inside and out. The folding roof works quietly and quickly.

Accessibility

Thanks to the car's size, there's little problem accessing the front of the car. The rear seats can be more troublesome to get at but the memory settings in the front mean the seats can be moved for easier access without losing the driver's carefully tuned settings.

Stereo and ICE (In car entertainment)

The 6 Series boasts an impressive sound system as standard, but there is a long list of upgrades available including satnav and music player connectivity. The iDrive system has been the subject of many refreshes with a view to making it more user-friendly.

Colours and trim

Inside, the car features exceptionally high quality materials and trim including leather seats that reflect heat, preventing them becoming uncomfortably hot in the summer. Colour choices are subtle and smart inside and out, complimenting the wood or brushed aluminium trim for the interior.

Parking

Front and rear sensors make manoeuvring the 6 Series easier, but as a big car with a low seating position it takes time before you're comfortable enough to back into tight spaces unaided. The rear wheel drive layout offers a good turning circle, however.

Spare wheel

Run-flat tyres fitted as standard.

Range information

Petrol engine options – 3.0-litre (320bhp); 4.4-litre (450bhp, 560bhp). Diesel engine options – 3.0-litre (313bhp). Transmission options: eight-speed automatic with wheel mounted paddle shift, seven speed Sequential Manual Gearbox for M6. Trim levels: SE, M Sport, M6.

Alternative cars

• Jaguar F-Type Beautifully styled and competitively priced soft-top GT, but no diesel option
• Mercedes-Benz SL Clever folding hardtop and impressive petrol only engine choices
• Porsche 911 Carrera Cabriolet Genuine sports performance and a highly desirable badge – for truly keen drivers
• Aston Martin V8 Vantage Roadster Left-field choice and only for those with deep pockets