Green NCAP, a fairly new vehicle assessment programme in Europe, has released its second round of results, rating five vehicles on their performance for “clean air and energy efficiency”.

The Mercedes-Benz C220d 4Matic and the Renault Scenic dCi 150 each achieved three stars, while the Audi A4 Avant g-tron – the first compressed natural gas (CNG) vehicle assessed by Green NCAP – also scored three stars. The outgoing Opel Corsa (in 1,0-litre petrol form) managed four stars, while the all-electric Nissan Leaf achieved a full five-star rating.

Scoring is divided into an overall rating, determined by two major categories: the clean air index and the energy efficiency index. Testing is performed both on the road and in a laboratory.

Interestingly, the C220d scored maximum points in terms of its clean air index, the same as electric vehicles. While electric vehicles produce no pollutant emissions, those of the Mercedes were below Green NCAP’s “high-performance threshold”. It was scored down slightly in terms of energy efficiency.

“It is clear that new diesel-engined cars, with proper calibration and effective after-treatment, can deliver extremely low pollutant emissions,” Green NCAP said.

The second-generation Leaf, meanwhile, scored full marks, although Green NCAP pointed out it test vehicles “tank-to-wheel and does not take account of the energy involved in generating electricity or producing fuel”.

“On this basis, electric cars currently offer the cleanest, most efficient motoring. Green NCAP hopes to be able to assess well-to-wheel emissions and, ultimately, the whole life-cycle of vehicles to give an even more realistic view to consumers,” it added.