Road Test: BMW 6 Series 635d Sport 2dr Auto

Interesting fact time. The facelifted BMW 6-Series, and specifically the 630i variant, is 23 per cent more fuel efficient than the last one. And emits 13 per cent less CO2, but pushes out 14 per cent more power.

I'm sorry, but those figures are simply staggering. The European Union says it wants to lower CO2 emissions by 20 per cent by 2020, but BMW has done more in the three years that the 6-Series has been on sale. Granted, the EU's task is ever so slightly more complicated, but even so, it's impressive stuff from Munich.

However, even all these numbers pale in comparison with the new engine in the 6-Series range - the 3.0-litre twin-turbo diesel that's been in everything from the 3-Series Coupe to the 5-Series Touring.

This is the only diesel you can get in this class and it comes in both of the body styles on the 6-Series. It's got a clever twin-turbo arrangement that works in tandem. At low revs, there's a small turbo which spools up quickly and lowers the amount of turbo lag from those points. Then at higher revs (about 2,000rpm) the bigger turbo takes over to give the top end punch.

You can rarely feel either kicking in, and the turbo entry point is much smoother on this engine than it is on the 123d. With the auto box that comes as standard here, the revs never dip below 1,500rpm anyway, so there's always instant power and torque. Speaking of which, it produces 282bhp and 427lb ft.

The 630i manages 34.0mpg, but the 635d does 39.2mpg, with only 190g/km of CO2. And in the cabriolet it's even 0.1secs quicker from 0-62mph at 6.6 seconds. This diesel engine is a cracker - it's refined enough to sit in a convertible (there's even a slight hint of a petrol-esque roar from the outside), as well as being punchy for in-gear acceleration.

For a reference as to how torquey it is, the car doesn't even set off from a standstill in first as it defaults to second when you're sat at traffic lights. Makes you wonder what the lowest cog is there for.

There's so little turbo lag and so much instant power that you get just as much enjoyment driving it in fully manual mode with the paddles, as you do wafting along in auto. That's some compliment, because normally diesels don't suit the manual 'boxes as the power delivery isn't quick enough to be pleasurable.

Here, on the other hand, you get to a corner, flick down a cog or two then just power through. This engine doesn't mind being revved and doesn't get harsh when you do so. Which is in contrast to the chassis.

I left it in manual for most of the drive, because I just found that suited the car more. In other words, it doesn't really do wafting very well. Granted, our car was on sport suspension with 19-inch wheels, but the ride is too firm, especially for a convertible. Relaxing it ain't.

Which is a shame because this should be better at cruising. The 6-Series is a big car and while it disguises its bulk really well, you're always aware there's something there. Much better to opt for the normal suspension, which will be more than capable of handling even the most committed back-road blast. It'll suit the engine and steering better as well, because it could do with a touch more feel through the wheel if you wanted the sporty drive the suspension promises.

The 6-Series facelift also gets some exterior and interior changes. Neither of which are at all noticeable so your neighbour will still think you're driving the old car. Oh no, wait. Apparently, the rear reflectors are now slightly different. And you get the eight programmable 'favourite' buttons in the dash, which at least makes the previously unfathomable iDrive slightly more fathomable.

This is being harsh though. The 6-Series is still an excellent car with the looks and chassis to compete at the top of this crowded segment. The replacement for the Mercedes SL isn't due until '08, so this facelift should keep the '6' ticking over for that clash.

But will people buying it want a diesel? I know guys who still refuse to go down the diesel route because they're convinced diesels are slow and smelly. So it's more than likely that the petrol variants, especially the 650i, will appeal to BMW 6-Series buyers more. Until petrol prices start pushing £1.50 a litre, anyway. Give it time...