Best in Class

Best in class:
smart fortwo CDI - 86g/km

Best of the rest:
Toyota iQ 1.0 - 99g/km
Nissan Pixo 1.0 - 103g/km
Suzuki Alto 1.0 - 103g/km
smart fortwo MHD - 104g/km

Because they are small and light, city cars have, on average, the lowest CO2 outputs of any class. Best of all is the smart fortwo diesel, which at 86g/km is by far the class leader with much lower emissions than the rest of the segment. Small car designers rely on attributes such as compactness and light weight to produce good fuel consumption and low emissions, as the cars are too cheap to have much in the way of high-technology. The smart micro hybrid drive is the only car here with anything other than a standard small petrol or diesel engine and sports a start-stop system to give it a low CO2 output.

Best in class:
Skoda Fabia Greenline 1.2 TDI - 89g/km
Volkswagen Polo 1.2 TDI Bluemotion - 91g/km
Seat Ibiza 1.2 TDI Ecomotive 3 Door - 92g/km

Best of the rest:
Ford Fiesta 1.6 TDCi Econetic - 98g/km
Renault Clio 1.5 DCI eco2 - 98g/km
Seat Ibiza 1.4 TDI Ecomotive 3 Door - 98g/km
Vauxhall Corsa 1.2 CDTI Ecoflex - 98g/km

There is now a huge scramble to get one version of each supermini below the all-important 100g/km of CO2 barrier that results in free road tax. No fewer than seven manufacturers now offer models just below the threshold, and the number can only grow. Superminis are too cheap to be viably equipped with hybrid powertrains, but improved engine efficiency and weight-saving measures will see steadily declining emissions. All the best cars in this class are diesel, incidentally.

Best in class:
Toyota Auris HSD - 89g/km

Best of the rest:
Seat Leon 1.6 TD Ecomotive - 98g/km
Audi A3 1.6 TDI E - 99g/km
Ford Focus 1.6 TDCI Econetic Start Stop - 99g/km
Volvo C30 1.6 TD Drive - 99g/km

There has been rapid change in this sgement recently. Toyota has launched the Auris HSD - basically a petrol Auris fitted with the Prius hybrid drivetrain. There have also been major developments in conventionally-powered models. The first sub 100 g/km small family car appeared in 2009 and there are now five such models to choose from. As stop-start technology becomes more widespread, this number will grow steadily. The lure of free road tax is so strong that no major manufacturer will want to be without a sub 100g/km model for much longer.

Best in class:
Toyota Prius T Spirit - 89g/km

Best of the rest:
Volkswagen Passat 1.6 TD Bluemotion - 118g/km
Peugeot 407 1.6 HDI - 129g/km
Renault Laguna 1.5 DCI - 130g/km
Toyota Avensis 2.0 D4-D - 134g/km

A new version of the iconic Toyota Prius gets the top spot miles ahead of everything else with 89g/km CO2. What is even more impressive is that it now features a 1.8 petrol engine, replacing the smaller 1.5 from the old car. With new DPF technology, the VW Passat 1.6 TD Bluemotion is best of the rest with just 118g/km CO2. It is also capable of over 62.8mpg, which is highly impressive for such a large car. In this segment the 100g/km barrier is going to be very hard to reach without electric assistance, but there will be steadily more models getting below the financially important 120 g/km threshold. Petrol engines, incidentally are now a small minority in this segment, accounting for under 20% of sales.

Best in class:
Volvo S40/V50 1.6 TD DRIVe - 104g/km

Best of the rest:
BMW 320 D Efficient Dynamics - 109g/km
BMW 318 D - 118g/km
Audi A4 2.0 TDI 136 E - 120g/km
BMW 320 D - 125g/km

Cars in this class are often performance oriented, but this bunch demonstrates that it's possible to buy these cars with very competitive CO2 emission figures. The Volvo S40/V50 has jumped to the top of the list thanks to Volvo's new DRIVe treatment, followed by the BMW 320d, fitted with the company's superb Efficient Dynamics programme. Audi has joined the low-CO2 party with its new A4 TDI E, rated at just 120g/km.

Best in class:
Volvo S80 1.6 TD DRIVe - 119g/km
Volvo V70 1.6 TD DRIVe - 119g/km

Best of the rest:
BMW 520 D - 132g/km
Mercedes E 220 CDI Blueefficiency - 137g/km
Audi A6 2.0 TDI 136 E - 139g/km

They're all diesels, and all turn in good CO2 figures given their size. For the first time in any segment, Volvo now has two models at the top of the class, thanks to its DRIVe programme. A figure of under 120g/km is a real breakthrough for an executive car. The BMW 520d has slipped down to third place, just ahead of the Mercedes E220 CDI Bluefficiency.

Best in class:
Audi A8 3.0 TDI - 174g/km

Best of the rest:
BMW 730 D - 178g/km
Jaguar XJ6 3.0 D - 184g/km
Audi A8 2.8 - 199g/km
Mercedes S 350 CDI Blueefficiency - 199g/km
Lexus LS 600H - 219g/km

Luxury cars produce decidedly more CO2 than executive cars, and often offer little more in the way of space, even though they are better equipped. The new Audi A8 3.0 TDI has just pipped the BMW 7 Series to the top spot. Just behind is Jaguar, whose lightweight new XJ is also exceptionally efficient. Audi's FSI engine is still up there though with CO2 emissions of 199g/km - the only petrol-engined saloon in any segment to appear in the list of lowest-emitting vehicles. This is thanks to clever aluminium construction, used in conjunction with direct injection and variable valve technology, to boost economy. The Lexus LS 600h has been left some way behind, despite its hybrid drivetrain.

Best in class:
Volkswagen Scirocco 2.0 TDI 140 - 134g/km

Best of the rest:
Peugeot RCZ 2.0 HDI 163 - 139g/km
Daihatsu Copen 1.3 - 140g/km
Alfa Romeo Brera 2.0 JTD 170 - 142g/km
Alfa Romeo Spider 2.0 JTD 170 - 142g/km

You wouldn't expect sports cars to return impressive CO2 figures, but this list shows progress is being made. Last year, only one car was below 160 g/km of CO2, but now there are quite a few models around 140 g/km or lower. The secret is a new range of efficient, but powerful diesel engines.

Best in class:
Audi A5 2.0 TDI 170 - 134g/km

Best of the rest:
Audi TT 2.0 TDI 170 - 139g/km
Mercedes CLK 220 CDI - 172g/km
BMW Z4 2.0 - 176g/km

The best cars here are diesels, but the Audi dominates much of this segment with new TDI versions of its TT and A5. Given that the A5 is substantially heavier than the TT, its 134g/km CO2 is all the more impressive. The Volvo C70 is in third place, but it is quite a long way behind the class-leading Audis. A hybrid Toyota coupe, due in a couple of years, will add interest to this class of car.

Best in class:
BMW X1 2.0 D - 136g/km

Best of the rest:
Volkswagen Tiguan 2.0 TDI 140 Bluemotion - 139g/km
Skoda Yeti 2.0 16V TDI 110E - 140g/km
Hyundai iX35 2.0 CRDI - 147g/km
Lexus RX 450h - 148g/km

Four-wheel drives are the enemy according to some environmental campaigners. And in many cases they are. But, not all four-wheel drives are high producers of CO2 as this list shows. None of the models in this list exceed 150g/km which shows just how far this segment has improved. The compact new BMW X1 is the lowest-emitting off-roader developed thus far, but there are a few other models close behind. The Lexus with its hybrid technology is only in fifth place, but that is because it is a size larger than all the other models in the list. It is certainly well ahead of any other off-roader of the same size. The vast majority of 4x4s bought today are diesels; in future we can expect more hybrid offerings.

Best in class:
Peugeot 5008 1.6 HDI 110 - 135g/km

Best of the rest:
Ford Galaxy 2.0 TDCi 115 - 152g/km
Ford S-Max 2.0 TDCI 115 - 152g/km
Seat Alhambra 2.0 TD Ecomotive - 159g/km
Volkswagen Sharan 2.0 TDI 140 Bluemotion - 159g/km

The new Peugeot 5008 has knocked Ford off its traditional top-spot in this segment. The 5008 has a remarkably low figure of just 135 g/km - although it has to be said that it is a little smaller than many rivals. Behind Peugeot and Ford come VW and Seat - but we can expect new models by 2011 which are likely to improve their performance. If cars like these are used with all seven seats in use, they make up some of the lowest emitting vehicles in the UK on a per-passenger basis. Virtually all people carriers sold in the UK are diesels, and there are no hybrids in prospect in the immediate future. Buyers in this class are very price-sensitive and diesels already give quite a good performance, given the size of the vehicles.