The best new 'green' cars

How can a car's "green" credentials be judged? By the amount of energy it consumes ­ the less, the better. But, with the global warming debate raging, we're more concerned about the emissions of carbon dioxide that result from burning carbon-based fuels. Nearly all cars do this, which is why, since 2001, the annual tax has been based on a car's CO2 emissions.

Cars made before that date are taxed as having either a small engine or a large one, the dividing line being set at a slightly strange 1,549cc. Newer cars fall into various emissions bands, and the Budget two weeks ago extended the banding and altered the figures. Now, cars emitting under 100g/km of CO2 in the official drive-cycle test attract no tax at all. Electric cars have always been tax-exempt, as they emit nothing except heat and the smell of warm insulation, but now the most frugal of fossil-fuel cars are similarly tax-free.

If your car emits between 100 and 120g/km, you pay a reduced rate of £50 a year if it runs on diesel oil, £40 if on petrol, and £30 if on an alternative fuel (which means LPG, CNG or bio-ethanol). Note that the diesel levy margin over petrol has risen from £5 to £10 because the Government still views diesels as more of a health hazard, even though many new diesel cars now have particulate filters. These should be rewarded, and are not.

Beyond 120g/km things are much as before, unless you buy a new "gas guzzler" ­ there's a new, top rate of excise duty, which means that a petrol car emitting 225g/km or more now attracts a duty of £210. Gas guzzler? That 225g/km figure still equates to about 30mpg on the " combined" (urban and open-road) test cycle, so there are plenty of much thirstier cars which are still getting off lightly.

So, what can you get that qualifies for zero road tax? Ha! The Chancellor flatters our green desires only to deceive. There's nothing officially available apart from the G-Wiz electric runabout with its restricted range. You could consider importing one of those nasty little Aixam microcars from France, but is the resulting purgatory worth saving £40 a year for?

But there's one further idea, and it's a good one. Smart has never officially sold the diesel version of its tiny two-seater here; too hard to convert to right-hand drive, the company says, and not enough demand to make it worthwhile for the low sales numbers likely to be involved.

But that was pre-Budget: has DaimlerChrysler UK now changed its mind? A telephone call to the press office revealed that it has not. This is an opportunity missed. But you can buy a Smart Fortwo CDI here in the UK if you really want to. It will be left-hand drive, but that's not much of a problem in a small Smart with a high seating position, and you quickly get used to it.

A quick trawl of the internet revealed models available at from £7,195 on the road, available from The Left Hand Drive Place in Basingstoke, Hampshire (01256 461 173). That's just £385 more than the cheapest right-hand-drive petrol Smart, and you get the warm glow of a minuscule 90g/km CO2 emissions plus average combined mpg of 83.1. In a surprising burst of generosity, the Chancellor doesn't even charge the £10 diesel penalty. The 799cc, three-cylinder turbodiesel gives spirited pace and (as from about now) it even comes with a particulate filter.

In the next band ­ cars which emit 101-120g/km of CO2 ­ there are just seven petrol-fuelled cars, but five of them do have some of the lowest emissions in the sector. Three of these are really the same car (the Citroën C1/Peugeot 107/Toyota Aygo near-clones) and their tiny, Toyota-made, three-cylinder engines emit just 109g/km ­ the same as the bigger, faster Honda Civic Hybrid, whose continuously variable automatic transmission and novel mechanism for closing the engine's valves appear to create a car that runs with extraordinary efficiency.

Toyota's well-known Prius, another petrol-electric hybrid, is even less of a global warmer with its 104g/km score, but the problem with these hybrids is that the promise of official combined mpg figures and CO2 measurements is seldom met in real-world use ­ because people just don't drive like that. It's on motorways that the hybrids most fall short of expectations, but if you do a lot of town driving or are never in a hurry they can be truly, rather than merely statistically, green.

The other two ultra-frugal petrol cars are also powered by a version of Toyota's 1.0-litre, three-cylinder engine: the Daihatsu Charade and its larger Sirion stablemate. Toyota's Yaris 1.0 misses the cut, however, because it is heavier and therefore thirstier.

Which leaves the many sub-120g/km diesels, many of which feel punchy and lively as well as sipping minimal amounts of fuel, however they are used. One of the most frugal, at 113g/km, is the Citroën C2 1.4 HDI, which isn't far behind the similar-capacity diesel C1 (109g/km, same as the petrol C1) despite being heavier and having 70bhp, against 55bhp from a similar but de-boosted version of the engine. It just shows that a diesel is more efficient when running a higher turbocharger boost.

Among that cars that just scrape into this low-tax band are the new Fiat Grande Punto 1.3 M-Jet diesel at 119g/km, the Peugeot 206 1.4 HDI SW estate car (120g/km) and one of the biggest-engined diesels in the sector, Citroën's C3 1.6 HDI (also at 120g/km).

And, as of June, we can add Peugeot's new 207, tested last week, to the list; it scores 120g/km in both 70bhp 1.4 and 90bhp 1.6 HDI diesel versions, so there's no real reason not to go for the bigger-engined model.

So, by buying a decent diesel, it's possible to transport a family and still pay just £50 a year car tax while enjoying genuinely minimal fuel consumption, even at speed on a motorway. That's the real secret of green motoring. Of all such cars, the most grown-up as well as the newest is that Peugeot 207 1.6 HDI. It also happens to be one of the most pleasing versions of the range. For once, everyone can be happy.