When deciding how to revamp Britain’s road tax system, politicians decided to use a measure they knew rather a lot about — hot air.

More specifically, the hot air that issues from every exhaust pipe. To be even more precise, despite the cocktail of noxious fumes that every car emits, they decided to pick on carbon dioxide as their villainous chemical combination of choice.

So, for good or evil, we now have 13 tax bands based solely on CO2 emissions, leaving car makers to busy away whittling down emissions and producing a range of ‘greener’ cars, that almost without exception bear an ‘eco’ badge, or something similar. Vauxhall’s offerings carry the ecoFLEX tag and the Astra version plays an interesting numbers game.

First, its CO2 emissions are 109 grams per kilometre which sounds boring, but that means you pay no road tax for the first year and only £20 a year afterwards.

Second and even more interesting, if you drive carefully, its 1.3-litre turbocharged diesel engine can deliver almost 70 miles per gallon in everyday driving.

Despite its modest size, the Astra’s engine delivers 95 horsepower, which makes the hatchback surprisingly lively.

OK, its gearing is arranged to deliver maximum fuel economy, which means it’s not the fastest off the line, but once up and running it proves to be a smoothly competent and refined motorway cruiser.

Like the rest of the best-selling five-door hatchback range, built at the Ellesmere Port plant in Cheshire, the new Astra has a much stiffer body than its predecessor, giving improved ride and handling, while also cutting noise and vibration, and uses specially set up steering settings to cope with Britain’s singularly tight twisting roads.

With its sleek silhouette, steeply-raked windscreen and sloping rear roofline, Vauxhall set their sights high with the new model, saying it aimed to appeal to buyers who would never have considered buying a car with a Griffin badge, even citing potential owners of upmarket cars like the Audi A3 as possible customers.

Prices range from about £16,000, with the ecoFLEX model coming in a couple of thousand above that. One last statistic designed to heighten the showroom appeal of the new Astra is that, like other Vauxhalls, it is now covered by a warranty lasting 100,000 miles.

Auto facts Vauxhall Astra 1.3 CDTi Exclusiv ecoFLEX

  • Price: £18,345
  • Insurance group: 9E
  • Fuel consumption (Combined): 68.9mpg
  • Top speed: 109mph
  • Length: 441.9cm/174in
  • Width: 181.4cm/71.4in
  • Luggage capacity: 13 cu ft
  • Fuel tank capacity: 12.3 gallons/56 litres
  • CO2 emissions: 109g/km
  • Warranty: 100,000 miles