what are the greenest cars?

• guardian.co.uk,

The Nissan Leaf prototype electric car. Photograph: Bryan Mitchell/Getty Images

Petrol

Most vehicles still have conventional petrol or diesel engines. Contrary to what you may read in the media, they won't be dying out any time soon – instead they will become more efficient. Better efficiency means lower emissions. Petrol engines are favoured for smaller city-type cars as they are lighter, more responsive for urban driving and cheaper to manufacture. There are new petrol engines, such as two- and three-cylinder units, coming on to the market that are extremely efficient.
Example: Nissan Micra DIG-S 68.9 mpg, 95g/km CO2

Diesel

Diesel engines are more economical than petrol engines, which means they also have lower CO2 emissions. If you mostly drive long distances, an efficient diesel engine may be your smartest option. The downside is that diesel engines can emit particulates that harm local air quality – moreso than petrol engines. The emissions are reduced if the vehicle has a diesel particulate filter (DPF). However, the technology that makes diesel engines cleaner bumps up the cost.
Example: BMW 520d EfficientDynamics Saloon 62.8 mpg, 119g/km CO2

'Green' petrol and diesel models

Many car manufacturers offer green versions of their conventional models. These are more fuel-efficient and so have lower emissions. This is achieved by improvements such as better aerodynamics, reduced weight, lower-resistance tyres, longer gearing, remapped engine management systems, stop-start systems and less load on the engine from ancillary items. Green models often have a price premium. So you need to work out if your mileage will bring large enough savings on fuel to pay back the extra investment. At least one manufacturer is applying efficient technologies to all its vehicles.
Example: Volkswagen Polo BlueMotion 80.7 mpg, 91g/km CO2

Hybrid Electric Vehicle (HEV)

A hybrid vehicle has a conventional engine and a battery-powered electric motor. The idea is that the engine is used most of the time, but in most hybrids the electric motor alone can provide power at low speeds. In urban use, this means a hybrid should offer reduced fuel consumption and CO2 emissions. At higher speeds, the engine and the electric motor work together to provide more power. Hybrid cars are generally more expensive than comparable non-hybrid ones, because they effectively have two powertrains.
Example: Peugeot 3008 Hybrid4 74.3 mpg, 99g/km CO2

Plug-in Hybrid Electric Vehicle (PHEV)

Plug-in hybrid vehicles will be here soon. These share similar technologies to hybrids plus you can plug these into an electricity supply to recharge the electric motor's battery. That means higher electric power capacity, a longer driving range on the electric motor – possibly more than 20 miles – and therefore lower exhaust emissions.
Example: Toyota Prius Plug-in Hybrid 134 mpg, 49g/km CO2

Electric Vehicle (EV)

We are at the start of an electric revolution. There are currently few electric vehicles on the market, but many manufacturers are entering the market. Electric vehicles do not produce any tailpipe emissions. If they are recharged using electricity from renewable sources, they are potentially the lowest emission vehicles you could use. Problems associated with electric cars are their limited range, the need to recharge and their high cost. However, they are ideal for short journeys in urban environments, where the number of recharging facilities is also expanding. Electric cars with the latest technology can be expensive, but their running costs are very low, and they can benefit from incentives such as congestion charge exemption and zero company car tax.
Example: Nissan LEAF N/A mpg, 0g/km CO2 if recharged from renewable energy

Extended-Range Electric Vehicles (E-REV)

Extended-range electric vehicles are electric vehicles at all times, with an electric motor powered by a battery, which is normally recharged by plugging in to an electricity supply. However, unlike pure electric vehicles, they have a range-extender generator on board – so if the battery charge runs out, then the generator starts up and recharges the battery, and continues to power the car. Extended-range generators are usually relatively small petrol engines and can provide up to an extra 300 miles range in some cases. Extended-range electric vehicles are even more expensive than pure electric vehicles because of the additional petrol engine. An E-REV is effectively the opposite to a hybrid – always electric with oil engine backup, as opposed to the other way round.
Example: Vauxhall Ampera 175 mpg, 40g/km CO2

Biofuel

Most fuel in the UK today includes a small percentage of biofuel, but some vehicles can be powered by biofuel in higher quantities, up to 90 or 100%. Biofuels are renewable; they can be produced from plant material or waste vegetable oil, for example. Some petrol engines can be adjusted to run on bioethanol or blends containing a high proportion of bioethanol, but bioethanol fuel is only available at a limited number of fuel stations. Some diesel engines can run on biodiesel or biodiesel blends, but these are also scarce, and can vary in quality.
Biofuels were in favour with the UK government a few years ago, until its environmental impacts became clear, such as rainforests being cut down to make way for biofuels. Since then much work has been done to ensure the sustainability credentials of biofuels,and if they are produced sustainably – for example from waste or non-food crops – then biofuels can become an important part of the overall transport fuels mix, providing a low-carbon fuel when other technologies may not be suitable. Although biofuels produce lower emissions, they do not necessarily improve fuel economy. Some mainstream manufacturers had cars a few years ago that could run on biofuels, but these are now in very short supply.
Example: Ford C Max FFV

Gas

Gas, such as liquefied petroleum gas (LPG) or compressed natural gas (CNG), can power some vehicles. Vehicles that run on LPG are usually dual-fuel conversions, ie they also run on petrol. LPG is a slightly cleaner fuel than petrol, and it is cheaper. LPG was promoted widely a few years ago, but with the recent emergence of other green car technologies it now has a much lower profile. CNG is used primarily for heavier commercial vehicles. Biomethane is renewable biogas derived from sources such as landfill sites, so it's very sustainable – it has one of the lowest carbon footprints of all transport fuels – but it is not widely available. As with biofuels, the lower emissions from gas does not necessarily translate to better fuel economy.
Example: Volkswagen Caddy EcoFuel Van

Hydrogen

Hydrogen is the ultimate future fuel, as its only emission is water. Some manufacturers are either producing or trialling cars powered by hydrogen, but there are significant hurdles such as the lack of a hydrogen refuelling infrastructure, the expense of making hydrogen-powered vehicles, and producing hydrogen sustainably.
Example: Honda FCX Clarity

Which should I choose?

Zero tailpipe emission vehicles are ideal for urban areas. Electric vehicles achieve this. They are available today, and the choice will increase with time. If you need a longer range than a pure electric vehicle offers, a hybrid or an E-REV may be the answer, as both are low emitters but don't have the range limitations of a pure electric.
If you want an economical vehicle for long distances, an efficient diesel may be the best bet. If you want a vehicle with a low purchase price and low running costs mainly for short urban journeys, with occasional longer distance driving, there are increasing amounts of highly efficient petrol cars. Whatever type of car you need, there will be class-leading models – find out what these are at green-car-guide.com.
• Paul Clarke is editor of green-car-guide.com