Hybrid vehicles produce the lowest carbon dioxide emissions during city driving, while natural gas cars do best on the highway, according to research institute Empa.

The Empa study, performed on behalf of the Swiss Federal Office for the Environment, investigated the carbon dioxide emissions of current hybrid and natural gas-fueled cars. Under mixed conditions (everyday driving), both types of vehicles offer reductions of up to 25 percent in carbon dioxide emissions compared to conventional gasoline models, the Empa engineers found.

In city driving, hybrids can achieve up to twice the efficiency of gasoline models. But during rural driving conditions, hybrids show little savings in terms of fuel consumption or carbon dioxide emissions compared to gasoline vehicles and none at all on the motorway, the study found. During highway driving, natural gas vehicles emit less carbon dioxide than hybrid cars, and during rural driving conditions, natural gas cars and hybrids are equally "clean."

The institute concluded that the total carbon dioxide emission levels of natural gas-powered automobiles are quite comparable to those of hybrid vehicles, and when rural and highway driving predominate, then they are in fact better.