The Smart Fortwo, VW Polo Blue Motion, Toyota Prius and Ford Focus C-Max CNG are among the most eco-friendly cars on the road, a research institute found recently.

Apart from analysing factors such as fuel consumption, noise and emission levels ecological criteria from production, logistics and recycling were applied by the Institute for Environmental Research based in the German city of Wuppertal.

So far the institute has awarded the environmental certificates to 15 car models in six categories.

In the small car category the institute praised the lightweight construction of the Smart Fortwo which contributed to fuel consumption of under five litres per 100 kilometres.

The VW Polo Blue Motion convinced the institute with its "progressive production" methods and exemplary recycling concept.

In the compact class, the Toyota Prius has set standards with low emissions as a result of the hybrid concept. The electric mode enabled almost silent cruising with emissions reduced to almost zero in some city driving conditions, it said.

In the van category the Ford Focus C-MAX 2.0 CNG scored with the concept of petrol and natural gas drive. It has a range of about 1,000 kilometres and switches automatically to petrol mode if no CNG filling station can be found. Good environmental standards were also set in production and logistics, the institute said.