Driving a car isn't the greenest activity in the world, but there are ways to minimise the impact your journey has on the planet and choosing the most environmentally friendly car you can is a very good start.

The chances are that you know broadly what type of vehicle you want, so rather than simply choose the 10 greenest cars on the market I've tried to pick the best in each class. That way, there should be something to suit almost any type of driver.

For each model I've included a rating, using a scoring system developed by WhatGreenCar? Unlike the government's new ranking system, which only assesses CO2 exhaust emissions, WhatGreenCar? also rates the environmental impact of each car's lifecycle (fuel and vehicle), expressing it as a score between 0 and 100 - the lower the score, the less the environmental impact. For comparison, an average small family car has a rating of around 50.

City car: Reva G-Wiz

Green score: 6
The Marmite of the green car world: you either love or hate the zero-emission electric G-Wiz. Fortunately for Reva, enough car buyers love it - in London at least where it is exempt from the congestion charge.

While it doesn't have any of the refinements of most cars, the G-Wiz delivers everything you need from a city runabout - as a result it is now the most successful battery electric car in the UK with nearly 1,000 already in use in the capital. Buy a G-Wiz and you'll join (should you want to) an impressive line up of celeb owners including Kirstin Scott Thomas and most recently Jonathan Ross.
Cost: from £6,999

City car: Smart fortwo LPG

Green score: 23
Although the fortwo design is nearly 10 years old, small is still beautiful. Smart first earned its green credentials by adopting innovative production methods and designing cars with recyclable body parts. The main reason why this model has such a good emissions rating is its small size - put simply, lighter cars generally have better fuel economy and lower emissions.

Although the petrol and diesel models are already low CO2 cars, the LPG bi-fuel fortwo does better (93gCO2/km) by using a low carbon fuel (liquefied petroleum gas). Can't find LPG near you? Don't panic - it is available at around 10% of UK fuel stations and you can always switch back to petrol when you need to.
Cost: from £9,400

Super-mini: Daihatsu Sirion 1.0

Green score: 32
By rights, the top super-mini slot should be taken by the Vauxhall Corsa, WhatCar's Car Of The Year 2007 - but the Corsa LPG (impact rating 29) is no longer available from Vauxhall direct [though you can convert your new or existing model at Millbrook; tel: 01525 408308) and the 2006 Corsa 1.0i ECO (31) has now been superseded by the 2007 range with the 1.3CDTi now the greenest model (35).

As a result, the petrol Sirion 1.0 litre pips the Corsa to the green post with its high fuel economy (56.5mpg) and low emissions (118gCO2/km). Although not perhaps as well built as the Corsa, it is worth noting that the Daihatsu model is built in Japan (considered a good thing by car connoisseurs), the company being part owned by Toyota.
Cost: from: £7,460

Super-mini: Mini Cooper D

Green score: 30
After having given its name to the mini category (aka 'city-car'), the new mini decided to shift up to the larger super-mini class so increasing emissions in the process. Now the Mini Cooper diesel has taken a step back by reducing emissions to levels more often associated with city cars. As a result, with CO2 emissions of only 104g/km (72.4mpg) the 1.6 litre Mini Cooper diesel is the cleanest car manufactured by BMW (who own Mini) - this puts the Cooper D in the lowest but one band B for road tax (£35 a year).
Cost: from £14,175

Small family car: VW Polo 1.4 BlueMotion 1

Green score: 30
Under its new BlueMotion badge, VW is launching green versions of all models in its range. The first to be launched in the UK this summer is the Polo 1.4 BlueMotion diesel which is a stripped-down version of the standard model (no air conditioning or central locking) and is fitted with a three-cylinder turbo diesel engine.

The result is very impressive fuel economy (74mpg) and carbon emissions below the 100g/km barrier (99g/km) which makes the Polo BlueMotion 1 road tax free. The Polo BlueMotion 1 is now available for ordering ahead of its arrival in the UK in October.
Cost: from £11,900

Small family car: Ford Focus 1.8 Flexi-Fuel

Green score: 37
Here's a Dr Seuss tongue twister for you - the FlexiFuel Ford Focus is the British bioethanol version of the most popular passenger car in the UK. True to its name, the Flexi-Fuel can run on any petrol/bioethanol blend up to 85% ethanol (E85). While its carbon emissions are 169g/km and its fuel economy is just 40.4mpg, what makes bioethanol environmentally attractive is that it can be made from biomass - usually wheat, sugarbeet or sugar - and is (in principle) a renewable fuel. Real world bioethanol reduces lifecycle CO2 emissions by 40-90% depending on the crop used. UK motorists may have some difficulty in getting E85 (only sold by around 14 fuel stations in the south west and east of England) - however this is one to watch as more UK outlets are planned for 2008/09.
Cost: from £14,145

Large family car: Toyota Prius

Green score: 35
There isn't much that hasn't been said about the Prius, the world's first and bestselling commercial hybrid car. Since its launch in 1997, the Prius has collected rave reviews and a loyal customer base.

With the current model Toyota has extended its market lead for hybrids, leaving most of its competitors behind. Indeed it plans to hybridise its complete car range. Hybrids work by using a temporary energy storage device (usually a battery), which enables the engine to be operated at close to its maximum efficiency. For the Prius this means CO2 emissions of only 104g/km, a fuel economy of over 65mpg and an improved driving experience.
Cost: from £17,780

Large family car: Volvo V50 1.6D DIESEL

Green score: 37
Although the Prius steals the show for a car of this size coming with its score of 35, the V50 shows what can be achieved by conventional fuels (in this case diesel). The V50 is Volvo's smallest estate (basically a slimmed down V70), but as you might expect, it has as big a focus on safety as do the larger cars in Volvo's line up. The 1.6 litre four-cylinder turbo charged diesel gives a very respectable fuel economy of 56.5mpg and relatively low CO2 emissions (132g/km) for a car in this class - remember an average small family car has an impact rating of 50 - the V50 beats this hands down.

Cost: from £18,000

Executive: BMW 318d Saloon

Green score: 42
As you would expect from BMW, there is no shortage of choice from the extensive 3-series saloon range - anything from the 318d (50.4mpg, 150gCO2/km), all the way up to the high-performance 335i (29.4mpg, 231gCO2/km). But that's the point - most model ranges will offer a greener model, often with less performance penalties than you might expect. The 318d will still give you BMW's trademarks (50:50 weight distribution and rear-wheel drive) - which means that even the greenest BMWs will still handle with a precision their rivals just can't match.
Cost: from £23,000

Sport: Vauxhall Tigra 1.3CDTi

Green score: 42
OK, so a sports car may not always be the greenest thing to drive, but as yet there's no law against it and, as even the folk at WhatGreenCar? agree, it can be fun. So, if you're going to get sporty, you could try Vauxhall's Tigra 1.3CDTi with its good fuel economy (47.1mpg) and CO2 emissions of only 124g/km. While, it might not be the fastest sports car off the grid (over 12 second 0-60mph), just remember that diesels have won the Le Mans 24-hour race two years running.
Cost: from £15,000

· For further information about green cars, visit whatgreencar.com