The price of green machines

New technology may be the future, but diesel and petrol cars are getting cleaner. Sean O'Grady picks some of the best examples

Today is United Nations World Environment Day, and to celebrate we've assembled for your inspection some of the cleanest, greenest and most economical conventional cars you can buy right now.

They're compact, certainly, but they all use basically the same trusty internal combustion technology that we've been relying on for personal transportation for the past century or so. In all cases, you can fit four adults into their dinky dimensions, albeit sometimes at a squeeze.

They can be stylish, too, as the three newest entrants to this sub-sector of the car market prove. The VW Polo Bluemotion has all the "mini tank" appeal of its siblings but has lost some kit and gained some aerodynamic aids and engine tweaks to make it the least-polluting conventional new car available. (By the way, it's called "blue" rather than "green" because of the German company's sensitivity to the political repercussions of the word green.)

Almost as impressive is the new Mitsubishi, which goes on sale later this summer and uses a tiny turbocharged engine to achieve a clever compromise between performance and economy. The Mini Cooper D also shows that you don't have to give up style if you want to help save the planet.

We're all in favour of allowing a thousand flowers to bloom when it comes to the new wave of ecologically sound – or sounder – motors, from electric and hybrid cars through to hydrogen fuel cells and biofuels. The Toyota Prius, Honda Civic Hybrid and Ford and Saab biofuel cars are all perfectly viable propositions. However, for many motorists the technology is still too new, or the cars are too wacky or insufficiently safe to consider for everyday use.

However, the Smart ForTwo is a special case in the class. It's a very capable machine, and actually pretty good in a smash, but it does only seat two.

The quickest and easiest way to go green is simply to downsize and drive more gently and economically. World Environment Day has been going since 1972, and it's interesting to note how cars use much less fuel and emit fewer noxious emissions than they ever did back then. Plus, they are faster, heavier and safer nowadays.

The only snag with some of these models is that the makers will force you to pay quite a high premium for them. Choosing a small diesel over a cheaper petrol version of the same car, for example, will cost you thousands more, yet you'll be unlikely to make up the investment in any realistic timeframe. It's difficult to see why this green premium should cost so much. Green cars can be good value, but they ought to be cheaper. We'll award a prize to the first carmaker to sell diesel and petrol versions of small cars at the same price (as Kia did, briefly), and on a sustained basis.

Mitsubishi i

The new kid on the block. The tiny 659cc three-cylinder engine is in the back, and it's turbo-charged to within an inch of its high-revving life. We like the clever, space-efficient design and the way it will go 54.6 miles on one gallon, dumping a mere 114 grams of carbon dioxide for every kilometre it travels. Not really for chucking around corners, though. Runs on petrol. About £9,500.

Toyota Aygo

Built alongside the near-identical Peugeot 107 and Citroë* C1 at a factory in the Czech Republic. An ideal city car, with three or five doors and lots of intelligent ideas. All three cars are cheap to buy and run, with petrol or diesel power. Is the £1,000-plus premium for the diesel worth it? It will do 69mpg (against 61mpg for the petrol) and both emit 109g/km of ......... CO2. We'd go for the 1.0-litre petrol. From £6,730.

Citroen C2

Superminis are too small and cheap to make it worth fitting leading-edge technology. Citroen does its best with a stop-start system on some versions of the C2, and there's a commendably clean diesel – 113g/km of CO2 and 69mpg, some 20mpg better than equivalent petrol models, which are £1,200 cheaper. Diesel, from £9,420.

Volkswagen Polo Bluemotion

The "Bluemotion" tag is being rolled out by VW to denote all the ultra-green versions of their cars, so Golfs and Passats will follow in due course. This Polo can, in theory, do London to Edinburgh on £20 of fuel, and emits just 102g/km of CO2, even less than hybrids. On official average, it will do 72mpg. Diesel, about £11,500.

Fiat Panda

Another very capable car, with flair and driver enjoyment thrown in. Fiat's advanced 1.3-litre "multijet dynamic" engine provides reasonable performance (just short of 100mph) and excellent economy. A gallon will take you 66 miles in this baby Fiat; that's better than the famous 500s of old, and translates into 114g/km of CO2. Petrol, £8,145.

Mini Cooper D

This is BMW group's cleanest-ever product, with an official average of 72.4mpg, translating into CO2 emissions of just 104g/km. It has a stop-start feature, so when you're at the lights you won't waste fuel and choke pedestrians. Why don't all cars have this? Diesel, about £14,000.

Daihatsu Charade

We're not sure about this one, as its looks aren't to all tastes, but it drives surprisingly well. This Japanese make specialises in smaller-engined vehicles, and the Charade shows how good they can be at making the most of a little: 58.9mpg and 114g/km of CO2 are the key numbers. Petrol, £6,460.