Choosing greener cars means you make immediate cash savings.

Not only do they have less impact on the environment, but they will return higher average fuel economy and cost less for a fleet to run than cars which have higher emissions.

Analysis of the Vehicle Certification Agency’s website and the Fleet News running cost database shows that the Smart Fortwo diesel is the lowest emission car currently on sale in the UK.

With combined fuel economy of 85.6mpg and CO2 emissions of 88g/km it betters not only the new Toyota Prius hybrid in second place, but is also slightly cheaper to run on a wholelife basis than its sister car, the fortwo mhd (micro hybrid drive) – a petrol model with stop-start technology.

Over a four-year/80,000-mile operating cycle, the Smart diesel will cost £12,368 to run, while costing drivers as little as £200 a year in benefit-in-kind tax.

However, the mhd version does offer the lowest possible benefit-in-kind tax bills of any car in the UK at £136 a year for a base rate taxpayer.

Small cars dominate the list, ranging from the two-seater Smarts through city cars such as the Toyota iQ, Citroën C1, Peugeot 107 and Toyota Aygo, to superminis like the Ford Fiesta and Seat Ibiza.

Three hybrid models make the list: the new Toyota Prius with CO2 emissions of 89g/km, the new Honda Insight at 101g/km and its sister car, the Civic IMA, on 109g/km.

All three also come with the added benefit of being exempt from the London congestion charge, while the Prius also avoids Vehicle Excise Duty.

The two Hondas pay a reduced rate of £15 a year as they enjoy a £20 reduction for being alternative fuel vehicles.

But there are surprises in the list. A trio of Volvo models – C30, S40 and V50 – all make the top 20 cleanest cars, blending both real-world practicality with ultra-low emissions thanks to a raft of fuel-saving measures such as stop/start technology.