There have been all kinds of Minis - striped ones, spotted ones, ones with Michael Caine or Twiggy in them. More recently there have been exciting new German ones, built by BMW in Oxford. But never before has there been a Mini with a diesel engine - at any rate, not an officially manufactured one, though you can bet there's more than one tinkerer out there who has adapted his Mini to run on diesel, just as you can bet there is another who has fitted up his Mini to run on two AA Duracells and a family carton of cranberry juice. It's that kind of car, or used to be.

Official Minis over the years were presumably too busy becoming classics of design to be overly concerned about tedious quotidian matters such as fuel consumption and alternative energy sources. It wasn't as if the Mini was an especially greedy consumer of petrol in the first place. The car was always about economy - economy of price, space and, certainly, comfort - and the old ones produced mileage figures which tended to boggle, say, the average Ford Granada owner. Indeed, even if you drove an old Mini everywhere in first gear with the choke full out and the handbrake on, you barely saw the inside of a petrol station from one year to the next.

Developing a diesel Mini, then, could be seen as a faintly needless project, the equivalent more or less of building a camel with a further hump. BMW didn't get where they are today, though, without watching the market and the fact is that sales of diesel cars have increased over the past two years to the point where this year they will account for a third of all new cars sold.

So here, finally, is the MINI One D. (New Minis distinguish themselves from old Minis in a number of significant ways, not least of all by referring to themselves in block capitals.) The MINI One D is the fourth variant of the new MINI, following the MINI One, the sharper, more dashing MINI Cooper and the mother of all new MINIs, the MINI Cooper S. To avoid accusations of being the boring, conservative brother of the family, the D has been given the Cooper S's go-faster side-sills. Inside it's kitted out like a MINI One, with the same bank of chunky toggle switches, and the big, kitchen-clock-style speedo, but in addition you get a free temperature gauge which the MINI One doesn't have.

So, anti-petrol on political grounds? Dismayed by the paltry 43 miles per gallon offered by the standard issue MINI One? Want to know what the temperature is? Then join the queue for the new diesel model which can increase that figure by nearly half as much again. (For the record, the MINI One D is the most fuel-efficient car built by BMW, though that's possibly not saying much, given the generally guzzly nature of the cars BMW build.)

Diesel's recent march to relative glory is significantly down to the fact that diesel engines aren't nearly the comedy routine that they used to be as recently as four or five years ago, when they sounded at tick-over like a set of metallic dustbins being joggled about in a waste skip, and could offer acceleration only to people who had taken the trouble to ring up and book the previous Christmas.

Nowadays a diesel engine's impression of a petrol engine can be, if not spot-on, then at least uncannily similar, especially if the diesel engine in question is engineered by BMW and based on an idea by Toyota, which happens to be the case with the MINI One D's turbo-charged, direct injection, 1.4-litre number. It is marginally slower and generally more grumbly than the petrol MINI One, but not to the point where anyone's going to complain much about it. And it will whisk you, your partner and whoever you manage to fit in the comically tiny rear seats rapidly and nimbly around the streets, to the point where you may even get the car into the sixth gear, which appears to be provided as standard.

And it will do all these things virtually for ever, without stopping. The MINI One D is good news for the planet, then. On the downside, it's potentially catastrophic news long-term for your collection of Nectar points via BP petrol station outlets. In a week of driving the MINI One D, I took it on a four-and-a-half-hour round trip from London to the coast and back, on many needless and variously traffic-clogged journeys of an urban and suburban nature, and yet somehow failed to press the needle below the half-empty point on the gauge.

It became something of a mission after a while, and I was left slapping the steering wheel in frustration come the end of the loan period as the meter again mocked my best efforts. My estimate was that the car would not require refuelling much before the next Budget, and I was only grateful I wasn't the owner of one and halfway through trying to complete a set of garage whisky tumblers.

The lowdown

MINI One D Price: £11,385 Top speed: 103mph Acceleration: 0-62 in 13.8 seconds Consumption: 58.9 mpg (combined) At the wheel: Paul McCartney On the stereo: Stockhausen En route for: Rye