How to buy a greener car

The pressure's on to cut motoring emissions. These cars claim to do just that – but do their eco-credentials stack up? Nigel Pollitt does the maths

Alistair Darling's Budget last week spelt out a carrot-and-stick approach to encouraging low-emissions vehicles and punishing the gas guzzlers. And the pressure is now on for car manufacturers and drivers to produce and buy vehicles that either consume less fossil fuel or none. Climate change and a looming oil shortage are forcing the environmental and pecuniary price of old driving habits through the roof.

The Lords are debating the UK's pioneering Climate Change Bill. The EU is discussing its controversial proposal that 10 per cent of road fuels within the EU zone should be biofuels by 2020. The Government wants the emissions levels of new cars to average 100g/km by 2020 and will seek EU-wide support for this aim.

From late October, Transport for London will exempt vehicles emitting up to 120g/km CO2 from paying the capital's congestion charge, while charging owners of vehicles emitting more than 225g/km £25 a day. And by 2010, first-year road tax for cars emitting more than 225g/km will be £950 – while cars emitting up to 130g/km will gain a 12-month exemption.

While drivers consider their options, car-makers are starting to produce vehicles that are, or could be, less damaging to the planet than their predecessors.

Old cars can, mind you, be relatively green. The longer you keep a car, the lower the CO2 impact of its manufacture. Our best hopes, however, lie with state-of-the-art and future cars, frugally propelled by fossil fuels, or powered by electricity, hydrogen and hybrid combinations in between. Biofuels may or may not loom large in the picture. First-generation biofuels can be very damaging to the environment and, while some argue the case for second- and third-generation biofuels, the jury is out on their overall environmental impact.

Sadly, manufacturers' claims about how eco-friendly their creations are can be misleading. Tiny electric cars and swanky bioethanol sportsters alike are constantly, wrongly, described as "emissions free".

How do you know whether a "zero emissions" electric car produces more or less CO2 at the power station than a hatchback? How do you rate hydrogen against bioethanol?

Until the manufacturers (with the honourable exception of Reva, maker of the G-Wiz) produce breakdowns of the ultimate greenhouse-gas emissions from all alternative fuels, knowing what is the best and most environmentally friendly vehicle won't be straightforward.

One day, a large share of our electricity may come from renewables, making calculations simpler for vehicles powered by electricity and from hydrogen – which is produced either from fossil fuel, or by using electricity to extract it from water. But, for the immediate future, the vast majority of electricity will continue to be generated using fossil fuel. So, here's a greenwash-free look at what's available, and at some of the exciting future models that could give the planet, and us, breathing space.

Low-emissions cars

Polo BlueMotion 1.4 TDi, Toyota Aygo 1.0, Mini Cooper D, Seat Ibiza Ecomotive, Citroë*C1, Peugeot 107, Smart ForTwo 1.0-litre, Ford Focus Econetic

How does it work?

Small, light cars yield 99g/km CO2 for BlueMotion and Ecomotive; all are under 120g/km.

How green is it?

Pretty green. Petrol and diesel cars convert fossil fuel to power more efficiently than electric cars charged from the grid.

Pros and cons

No road tax, no London congestion charge from 27 October. But diesel produces dangerous hydrocarbon particulates.

Can I buy one?

Yes. Polo BlueMotion from £12,120; Toyota Aygo from £6,945; Citroë*C1, £6,990; Peugeot 107, £7,195; Seat Ecomotive launched next month, Econetic follows.

The all-electric car

G-Wiz, Piaggio Porter MPV, Nice Mega City Car

How does it work?

Electric motor runs on batteries, recharged from the mains.

How green is it?

Very, potentially. Zero on-road emissions, but the 3bhp G-Wiz outputs 63-66g/km CO2 at the power station.

Pros and cons

Cheap insurance. Free charging points in London. No road tax or congestion charge. Electricity from 1p to 1.6p per mile, but range is limited.

Can I buy one?

Yes. G-Wiz, £8,895; Piaggio Porter MPV, £18,794; Nice Mega City, £10,847; Mega City, £11,347.

Hydrogen combustion car

BMW Hydrogen 7

How does it work?

Dual-fuel vehicle burns hydrogen or petrol with no loss of performance. BMW is using this fast, 260bhp V12 flagship musclecar to sell hydrogen combustion.

How green is it?

Hydrogen is made from fossil fuel, or from water by using electricity, thus releasing CO2. There isn't yet enough "green" electricity to power a worldwide hydrogen-based transport industry. BMW claims this car is "virtually emissions-free" at 5g/km CO2, but a figure including CO2 from hydrogen production would be higher. Using petrol, it's a 332g/km beast.

Pros and cons

Main on-road emission in hydrogen mode is water/water vapour. It can go for 125 miles on a tank of hydrogen, and up to 435 miles with the petrol. But burning hydrogen gives off smog-producing nitrogen oxides. And you might be waiting some time for hydrogen at your local BP.

Can I buy one?

No; you wait for BMW to invite you to have one of only 100 Hydrogen 7s on loan. Brad Pitt (above) is among the chosen.

Fuel cell car

Honda FCX Clarity

How does it work?

A hydrogen fuel cell generates electricity by splitting protons from electrons. The waste product is water. A tank of hydrogen feeds the 100kW cell.

How green is it?

Depends. Fuel cells guzzle hydrogen more efficiently than combustion, and are zero-CO2 on the road – but how is the hydrogen produced? Honda claims that, all in, the Clarity produces "less than half" the CO2 of a conventional car. Recapturing power from braking is very eco. Honda claims a 68mpg energy equivalent.

Pros and cons

Go 270 miles on one tank of hydrogen. Zero CO2 on the road. But there's no hydrogen supply infrastructure.

Can I buy one?

The Clarity will be leased to a few Californians later this year.

'Series' or 'plug-in' hybrid

Opel Flextreme, an MPV-style electric concept car.

How does it work?

It has an electric motor powered by lithium-ion batteries and a turbodiesel generator to recharge the batteries when the car is moving. Or you plug it in. The electric motor propels the car, the diesel unit is not connected to the wheels.

How green is it?

Very, on paper. GM claims it would emit about 40g/km CO2 – half that of the greenest diesel, the Smart ForTwo (not sold in the UK). Electric motors are highly efficient, and the diesel generator can spin at optimum revs.

Pros and cons

Go 34 miles on the battery; then use the generator for up to 445 miles. Should save you a lot at the pumps. Video cameras and widescreen display instead of rear-view mirrors. But try explaining to the AA man that the diesel isn't connected to the wheels...

Can I buy one?

Not until GM is happy with the battery. But you can watch the Flextreme in simulated action at www.youtube.com/watch?v=kMv-DOznBe8.

Bioethanol car

Saab 9-5 2.0t BioPower, Ford Focus and C-Max Flexifuel, Volvo C30 1.8F SE Sport

How does it work?

Bioethanol vehicles have internal combustion engines and run on various ratios of bioethanol to petrol, or just petrol. E85 fuel is 85 per cent bioethanol, 15 per cent petrol.

How green is it?

Jury's out. Some "first-generation" biofuels can, taking all effects into account, cause higher CO2 emissions than fossil fuels. Bioethanol is less potent than petrol, so mpg isn't the best. On the road, the Ford Focus Flexifuel produces 169g/km CO2, the Volvo 168g/km. These figures may be reduced by fuel crops "fixing" CO2, but by how much, after all impacts are considered, is highly debatable.

Pros and cons

Reduced road tax. But E85 is only available at some branches of Morrisons.

Can I buy one?

Yes. Volvo C30 1.8F SE Sport, £19,295; Saab 9-5 2.0t BioPower, £26,710; Ford Focus Flexifuel, £14,435.

Petrol-electric hybrid

Honda Civic Hybrid, Toyota Prius, and three Lexuses, such as David Cameron's GS 450H

How does it work?

Petrol engine is coupled with a generator, battery and electric motor. Electric motor assists petrol engine or can take over from it. There's no plug; the electric charge comes from braking and deceleration energy. Complex engineering controls and couples the power sources.

How green is it?

Not bad. Civic Hybrid emits 109g/km CO2, and the Prius 104g/km, in government tests that play to their strengths, especially in urban cycle. On the road, the figure would be 135 to 155g/km, based on the mileages obtained by reviewers. Lexus GS 450H emits 186g/km, but it's an executive cruiser.

Pros and cons

No London congestion charge. Honda and Prius are road tax band B. But the engineering increases the cars' weight and the CO2 impact of their manufacture.

Can I buy one?

Yes. Civic and Prius, from £17,775; Lexus GS 450H, from £39,965.