While the aim of WhatGreenCar? is to provide ratings for all new cars and promote the greenest models, we often get asked to identify the greatest environmental offenders. Although the usual suspects do appear in the 'most wanted' list, there are a few surprises, and lessons to be learned that can help car-buyers of all persuasions make greener choices should they so wish.

So, without wishing to offend too many manufacturers or existing owners, here is a selection of some of the top 10 new 'gas guzzlers' by vehicle class. For each model we have included the WhatGreenCar? (WGC?)rating that indicates the total 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. We normally top out the rating at 100, but for this article we make an exception!

Super-mini: Mini Cooper S Convertible (automatic) - 56

Small family car: Mazda 3 2.3 Turbo MPS - 62

Small family car: VW Golf 3.2 V6 4Motion - 68

Large family car: Saab 9-3 Saloon 2.8 TS (automatic) - 69

Large family car: Vauxhall Vectra 2.8i V6 Turbo (automatic) - 70

Executive: Audi A8 W12 quattro Tiptronic Sport - 94

Executive: BMW M5 Touring - 96

Sports utility: Hummer H3 3.7 L5 (automatic) - 92

Sports utility: Cadillac SRX 4.6 V8 (automatic) - 105

Sports car: Lamborghini Murcielago 147 Coupe - 132

Given its reputation, it is surprising that a Mini should appear in this list at all - all the more so as one of its sister models, the diesel Mini Cooper D, is one of the greenest cars in this class (WGC? rating 30) . This illustrates the first lesson regarding choosing a green car - model variants within a vehicle range will vary widely in their environmental performance. That said, if you want a super-mini that leaves a disproportionately large environmental footprint, this is a car for you. With CO2 emissions of 208g/km and a fuel economy of only 32.5 miles per gallon (mpg), the 1.6 litre Mini Cooper S will also help burn a hole in your pocket - the Cooper S is in the highest but one band F for road tax (£205 a year). Do the right thing - buy a Cooper D instead!It must take a great deal of skill and determination on the part of auto engineers to make a small family car like the Mazda 3 have such high CO2 emissions that it falls into the highest road tax band G, more usually associated with the 'Chelsea tractor'. This not-so-hot (eco-wise) 'hot hatch' emits an astonishing 231gCO2/km and only gives 29.1mpg of fuel - the emissions being well above the average of 160gCO2/km for this class. What makes this eco-unfriendly model worthy of note is that all the other Mazda 3's in the line up have excellent fuel economy and relatively low emissions - the diesel 1.6 TS 110ps has a WGC? rating of 37 and CO2 emissions of only 128g/km.If you buy the 4-wheel drive 3.2 litre VW Golf, street cred and pulling power (in both senses) will probably be more of an issue than its green credentials. If your passengers knew that the 4Motion emits a whopping 255g/km CO2, this might detract from any positive impressions, as may the fuel economy of only 26.4mpg. Although you don't have to downsize all the way to the best-in-class VW, the Jetta 1.9 TDI with particulate filter (WGC? rating 38), you might want to consider at least switching to the 2.0 litre TFSI DSG GTi (WGC? rating 50).While one part of Saab is pioneering the use of bioethanol (e85) that can cut life-cycle CO2 emissions by up to 70%, another design team has produced a model with significantly higher emissions than the average UK family car (WGC? rating 56). With a fuel economy of 26.2mpg and CO2 emissions of 259g/km, you are not going to get much sympathy from the chancellor either as you'll be (quite rightly) clobbered with the highest rate of road tax (band G, £300 and probably rising). So if it's got to be a Saab, check www.saabbiopower.co.uk to see if there are any e85 stations near to where you live. If there are, at least consider the 2.0 litre BioPower option. You will lose 80 bhp, but you could save up to £4k on the list price. Oh yes, and if you happen to be in Stockholm, you won't have to pay the congestion charge either.The Vectra 2.8i is another good example of how increased power leads to more emissions. Using petrol and being a V6 doesn't help either. With an emission rating 25% above the average in this class, a fuel economy of 25.9mpg and CO2 emissions of 262g/km, this model is in road tax band G. OK so it's got plenty of 'va-va-voom', but if you could settle for the a 2.2i 16v Direct Hatch, you'd still get 'va-voom' and your WGC? rating would fall to a relatively respectable 48 (CO2 180g/km). If you are happy to settle just for an everyday 'voom', the Vectra range also includes some very clean diesels - the 1.9 CDTI 16v comes in at 44.With a 6.0 litre engine packing over 440bhp, it's not a great surprise that this particular gas guzzler (19.2mpg) is in the Top 10. This A8's CO2 emissions of 353g/km exceed the average carbon emission for this class (202g/km) by 75%. Not that all the models in the A8 range have such a large environmental impact - both the petrol 3.2 litre FSI and the diesel 3.0 litre TDI (with particulate filter) manage to achieve WGC? ratings in the low 60s (CO2 emissions 227-237g/km). However, ironically, only the 3.0 litre TDI without a filter manages to beat the 225g/km CO2 threshold by one point and avoid being stung by next year's proposed £25 congestion charge for band G vehicles.If you use a company car and you drive this particular 5.0 litre model, then either the recent reforms of the company car tax system have completely passed you by or your company is doing so well that profit is no longer your overriding motive. At 18.8mpg and 361gCO2/km, the M5 Touring leaves a larger environmental footprint than a Hummer (see below)! Luckily BMW are making it increasingly easy for you to own all the refinements you would expect from the brand and be green - the new 5 Series 520d Touring (with particle filter) comes in with a WGC? rating of just 38 (CO2 136g/km).You couldn't do a Top 10 'gas guzzler' list without including the Hummer. And with a mass of over 2.2 tonnes, staggering CO2 emissions of 346g/km and a WGC? rating of 92, it sure doesn't do the environment much favours at all. What is surprising is that, as you can see from some of the other cars in this list, it's not the most environmentally frightening car on the road by a long way. That said, there's plenty of room for improvement - by switching to other well known SUV models such as the (not insubstantial) diesel BMW X3 Series E83 X3 2.0d (WGC? rating 47) or the petrol Dodge Caliber 1.8 (WGC? rating 48), you could easily cut your environmental impact by half.It's BIG, it's a Cadillac, it's 4.6 litres, it's a V8, it's an automatic, it weighs over 2 tonnes and it uses petrol (not known for its low CO2) - ergo its got high emissions to say the least - 396g/km of pure CO2. It's actually worth repeating this figure again but in better units - it emits a kilogram of CO2 every 1.5 miles! It burns fuel so fast, that its imperial (16.9mpg) and metric fuel economy figures (16.7 litres/100km) are almost numerically equivalent. In real money, this car costs around 26p/mile in fuel alone - almost as much as the price of a local bus ticket. Our verdict? Do the environment a favour and buy a Hummer instead!Lamborghini don't just make sports cars, they define what we think and desire a sports car should be. However at this level, design and performance come at an environmental price - generally speaking, engines with high power emit plenty of emissions. Just take a look at the stats - 631bhp, 495gCO2/km and 13.3mpg. If you are going to drive one, certainly celebrate engineering at its fastest - but don't use it to nip down to Tesco for a pint of milk or drop the kid (singular as there isn't room for any more) off at school to impress the other parents. Although if you already own one, your little one is probably already at school where the air is clean ... in Switzerland!

· For further information about green cars and the latest Top 10+ green cars (by class), visit www.whatgreencar.com