The firm assessed the overall figures for the 10 types of vehicles showing the biggest CO2 improvement auctions between 2010 and 2014. The figures show that coupes in the Volkswagen Scirocco class delivered a reduction of 31% from an average of 245g/km to 168g/km.

Other types of vehicle that also saw a major improvement were MPVs, lifestyle 4x4s and convertibles.

Rupert Pontin, Glass's head of valuations, said: ‘This is a good way of seeing exactly how much CO2 figures have been driven down among used cars in recent years and the large degree of progress that has been made.

‘It is especially pleasing to see that some of the biggest and fastest vehicles at auction, previously the worst carbon offenders, have seen the most significant improvements. There was a time when it was difficult to see how coupes and large 4x4s would ever perform acceptably in CO2 terms but manufacturers have performed minor engineering miracles.

‘However, if car manufacturers are to meet future targets, especially the commitment to have a fleetwide average output of 95g/km by 2020, then further strides will need to be made and we would expect to see these figures reduce further in quite a dramatic fashion in the coming few years.’

The overall figures for the 10 types of vehicles showing the biggest CO2 improvement are:

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