If you think that two-wheel-drive vehicles are cleaner than four-wheel drives, then some recent research may well prove you wrong.

The latest real-world test data from Euro 6 cars added to the comprehensive EQUA Air Quality (Aq) Index reveals that the more capable models actually produce less nitrogen oxides. In total, 48% of the four-wheel-drive models tested fell in the lower EQUA Ag A-C rating, compared to just 28% of two-wheel drives. Also, a total of 14% of Euro 6 compliant four-wheel-drive vehicles were clean enough to receive the EQUA Aq A grade, compared to 10% of two-wheel drives.

Developed by Emissions Analytics, the world's leading provider of real-world emissions measurement, the EQUA Index is totally independent of car manufacturers, trade associations and government. It looks at fuel consumption, carbon dioxide, carbon monoxide and nitrogen oxides. They use easy to understand ratings and have a database of over 60,000 vehicles.

Nick Molden, CEO and founder of Emissions Analytics, said: "These latest results are very encouraging, and show that it really is possible for large, four-wheel-drive vehicles to top the tables for low emissions. Cars, like the latest Porsche Panamera, are now among the best - which is a huge achievement when you consider that the comparable model or the previous generation Panamera was rated under the lowest grade. This underlines that, with the proper development work, all cars can cut harmful NOX emissions."

Those that came out favourably from the recent rating include the Porsche Panamera 4.0 V8, the Audi Q2 2.0TDI Quattro and the VW Tiguan 2.0 TDi 4Motion. They all received EQUA Aq A grades. It underlines the vast improvements that some manufacturers have made in cutting harmful emissions.