Forget the hybrids and electrics, diesel still has a lot to offer eco-conscious drivers

Audi A1
Audi would like its latest addition to be seen as a new category of small but luxurious car. While that's overstating the case, the A1 feels more lavish inside than many similar-sized cars. Inside, it offers the typical high-quality Audi cabin, complete with desirable gadgetry beyond many cars of the same size, including a pop-up sat-nav and the same sound system options as elsewhere in the range.

An electric version has been mooted, but for now the 1.6TDI offers the best fuel economy. At just under 60mpg, it is better than even the smallest petrol engine, a 1.2TFSI that drives the car beautifully and achieves just shy of 50mpg. The diesel also emits 99g/km of CO2, compared to 118g/km for the petrol version. Audi offers a wide range of customisable options, so expect to see A1s in strange colour combinations on the streets later in the year.

Mini Countryman
When it first emerged, the new Mini upset purists because it wasn't really a Mini at all (it's the same height as a Ford Fiesta). But it remains the market leader in the "small and sexy" segment. And while newer entrants, including the A1, Fiat 500 and Citroën DS3, are pushing hard, the Mini retains its place as the acceptable face of downsizing. BMW may be eagerly piloting its Mini E electric version, but a more realistic option is to take advantage of the BMW diesel expertise and opt for the Mini One-D diesel. For those keen on a bit more space, Mini has launched its Countryman, the first proper four-seater Mini. If that's some sort of carmaker tautology, further confusion arises over the fact that this most urban of cars also comes with four-wheel drive. Its pimped-up looks will delight and enrage in equal measures and while purists will howl, it will doubtless fly out of showrooms.

Citroën DS3
Another contender for coolest new small car is this gorgeous offering. It marks a new phase—indicated by a new corporate logo—for the French maker. French firms have delivered several exciting-looking cars in recent years, with Peugeot's RCS collecting several design awards. The DS3 must surely be in line for plaudits in the future. Its beguiling looks are added to by a wealth of clever engineering, and courtesy of a small diesel engine developed alongside BMW, it emits just 99g/km of CO2. It is certainly worth a drive for anyone keen to downsize and eager to do their bit for the environment without sacrificing anything in the way of aesthetics.

BMW 520d Saloon
Regardless of the pressures to downsize and go electric, some drivers will be reluctant to give up the comfort and security of a larger car. One of the most successful fleet cars in history has been BMW's 5 Series—the car's sixth iteration is just entering UK fleets. Good news for fans under pressure to go green is that the firm's EfficientDynamics programme has won approval for greening the range. The entry-level diesel here sums up why. With Auto Start Stop and other clever gadgetry, a manual transmission model will clock up just 129g/km of CO2 and return not far off 60mpg. It is about as environmental as a big saloon can get. It may be a little sluggish from 0 to 60mph (at over eight seconds, it's three seconds slower than the top-of-the-range 5-series) but it will power you to a top speed of 140mph—should you be somewhere where that is feasible and legal.

Mercedes E220 CDI
BlueEfficiency Estate
For those road warriors who still need to cart around endless amounts of sample material, an estate is often the only option. For those clocking up serious mileage, personal safety is always as much of an issue as concern for the rest of the planet. Here it is hard to go wrong with a Mercedes E-Class, for many years a staple of the fleet list. The latest diesel range adds significant advantages in terms of performance and economy. With safety features including Attention Assist—which alerts drivers if they are showing signs of nodding off at the wheel—it looks after the driver, and with fuel economy of just under 50mpg and emissions of 150g/km of CO2, it looks after everyone else as well.