How many years will it be until it is possible to buy a family saloon that will accelerate as fast as a Porsche Boxster and have an electronically restricted top speed of 155mph, yet that will use less fuel and emit lower levels of CO2 than a 1.4 litre Fiat?

The answer is none. This BMW 330d does that and is on sale now. It is the facelifted version of the 3-series that was introduced three years ago, but it is the engine — rather than the more visible styling changes — that are most impressive.

In fact its performance seems so contradictory to its economy that you can cover 100 high-speed miles and think the fuel gauge has broken. It hasn’t: by throwing the technological kitchen sink at its 3 litre, six-cylinder diesel engine — including its third-generation common-rail fuel-injection system — BMW has achieved what experience and basic common sense would suggest is impossible.

The unit produces 245bhp but returns 49.6mpg in the combined cycle and emissions of just 152g/km. When BMW fits it with its Start-Stop software (which is standard on cheaper, four-cylinder models but at the moment unavailable with engines of six cylinders or more), the economy and emissions numbers will improve to more than 50mpg and less than 150g/km. This would qualify it for the same band C, £120 tax disc as the cheapest Ford Fiesta.

Better still, this package appears to come without strings attached. The motor starts with a purposeful growl, delivers the bulk of its shove from little more than idling speed and will continue to rev past 5000rpm when most diesels are ready to call it a day not long after 4000rpm.

However, these landmark standards are not quite maintained by the rest of the car. Indeed it seems odd that so much of the budget has been spent improving still further an engine that was already the runaway leader in its class, and particularly so when the greatest threat to the 3-series is now posed by the new Mercedes-Benz C-class. For while the Merc looks cool and classy, the most charitable thing I can think to say about the modest front and rear styling tweaks meted out to the BMW is that they make it look slightly less dull than before. But not much.

Inside, the principal change does nothing more than correct a mistake BMW has been making since the turn of the century. At last BMW has replaced its iDrive operating system and if imitation really is the sincerest form of flattery, Audi should by now be blushing a nice shade of scarlet. Instead of all functions from navigation to entertainment being accessed via one central control wheel, you now press a button to call up your desired menu and only then do you explore that menu by using the wheel like a computer mouse.

It’s an approach pioneered by Audi’s MMI (multi-media interface) and BMW should be applauded for swallowing its pride and acknowledging its superiority. At any rate the result, while still some distance from perfect, is at least easy to understand and operate. Within one day I was using it more intuitively than after many years’ exposure to its predecessor.

Big changes were never likely for an interim update like this, so there’s no surprise in learning there’s still not enough room in the back, the interior remains unimaginative in style and inexpensive in appearance and there’s not enough storage space in either the cabin or the boot.

You should be cautious, too, before spending nearly three grand extra on the more dashing-looking M Sport model. You may like the looks, but the ride quality is ruined by its overly firm suspension, the fatter tyres make a dreadful noise on coarse surfaces and I didn’t get on at all well with its figure-hugging sports seats. A basic SE version may look more boring, but it’s a better and cheaper car by far.

A disappointing car with a great engine then? Far from it. The real reason BMW has decided — engine apart — to do no more than titivate this 3-series is that it knows perfectly well it doesn’t need to in order to maintain its class-leading position. The threat from the C-class is not to be underestimated, but the Mercedes is nowhere near as good to drive as the 3-series, while the new Audi A4 has turned out to be a sizable disappointment. The 330d is thrillingly quick when required to be, and admirably nimble too, but also refined and easy to live with the rest of the time.

In fact, I have no hesitation in calling the 330d the most desirable small saloon for people who live in the real world.

BMW 330d SE

ENGINE 2993cc, six cylinders

POWER 245bhp @ 4000rpm

TORQUE 384 lb ft @1750rpm

TRANSMISSION Six-speed manual

FUEL/CO2 49.6mpg / 152g/km

ACCELERATION 0-62mph: 6.1sec

TOP SPEED 155mph

PRICE £30,930

TAX BAND D (£145 a year)

VERDICT Still the one to beat

RELEASE DATE Out now