Engine: 3.0-litre in-line six-cylinder turbo-diesel

Transmission: eight-speed automatic

Power: 313 PS at 4,400 rpm

Torque: 630 Nm between 1,500 and 2,500 rpm

Fuel consumption (combined cycle): 49.6 mpg

CO2 emissions: 149 g/km

Top speed: 155 mph (artificially limited)

Acceleration (0-62 mph): 5.4 seconds

Price: £63,900 (6-Series Gran Coupé range starts at £61,390 for the 640i)

What do you get if you stretch the wheelbase of BMW 6-Series Coupé by 113mm and add an extra pair of doors for the rear passengers? According to BMW the answer is not, as you might suppose, a 6-Series saloon but a four-door coupé – or Gran Coupé to give it the name under which it will go on sale in June.

The idea of a four-door coupé isn't a new one. Mercedes didn't really invent the idea, but it certainly produced the first modern car of this type with the original CLS in 2004. Volkswagen came up with a half-price near copy, the Passat CC, and in terms of style, Jaguar's handsome modern four-door cars, the XF and XJ, are also probably more directly comparable with the CLS than with Mercedes' E-Class and S-Class saloons. BMW expects the 6-Series Gran Coupé to compete most directly with the second-generation 2011 W218 CLS, the Porsche Panamera and the Maserati Quattroporte.

Three engine options are offered – a 320PS three-litre petrol straight six (640i), a 450PS 4.4-litre petrol V8 (650i) and a 313PS three-litre diesel straight six (640d). All cars are fitted with eight-speed automatics. The diesel is expected to account for some 80% of Gran Coupé sales – it is already the most popular choice on the two-door 6-Series Coupé and the CLS and Panamera are offered with diesel too.

So what's the 6-Series Gran Coupé like? It feels and drives exactly as you would expect a slightly stretched four-door 6-Series Coupé to feel and drive, which is to say that it is excellent. But let's focus for a moment on the areas where the two-door and four-door cars differ most. Thanks to that stretch in length, the Gran Coupé just breaks the five-metre barrier that marks out really long cars from the rest, and because the new car's growth is concentrated on the wheelbase, it benefits the passengers' legs rather than their luggage. The Gran Coupé's cabin, which shares the two-door's dash and trim, has quite a cosy coupé feel, but there's more than enough space where it counts, thanks in part to a subtly raised roofline.

I drove the 640d and it's easy to see why so many 6-Series buyers plump for the diesel, which is mostly very quiet - except under load, when it emits a slightly deeper version of the traditional BMW six-cylinder engine note without ever sounding unpleasantly rough or dieselly. Performance is very strong – the 640d will get to 100km/h (62mph) in 5.4 seconds, and like the petrol models, is artificially limited to 155mph. BMW has focused heavily in recent years on reducing emissions and fuel consumption on even its sportiest cars under its Efficient Dynamics programme, and the 640d is no exception, achieving an outstanding 149g/km and 49.6mpg in the official tests.

But good as the Gran Coupé is on the road, I suspect that like the CLS it will sell mainly on its looks; after all, the impressive engines and technology these two models embody are available for less money in BMW's and Mercedes' staider saloon ranges. Which is the better car is mainly a matter of taste. The second-generation CLS doesn't cut quite the same dash as the 2004 original but is still far more of an attention-seeker than the Gran Coupé, which should be just the thing for seekers after understated style with £60,000 to spend on their next car.

Search for used cars