First, we should point out that while gasoline-electric hybrids and diesel vehicles both achieve impressive highway fuel economy, hybrids excel in city driving. For example, the Honda Civic Hybrid and the diesel Volkswagen Jetta TDI have similar highway fuel economy (45 and 40 mpg, respectively) but the Civic Hybrid gets 40 mpg in the city versus 29 mpg for the Jetta. In addition, diesel and gasoline have different chemical compositions and, thus, different emissions when burned. Over the course of the entire fuel cycle (i.e., from the time the fuel is extracted at the wellhead to the point at which it is emitted from a vehicle’s tailpipe), burning a gallon of diesel emits roughly 10 percent more global warming emissions than a gallon of gasoline.

As a result of the combined differences in chemical composition and fuel efficiency, the Civic Hybrid emits an estimated 4.4 tons of heat-trapping emissions for every 15,000 miles of driving (a typical amount of mileage for U.S. drivers in one year), while the Jetta releases 6.4 tons—not unlike many other cars on the road, such as the non-hybrid Ford Focus (6.8 tons). Diesels, while significantly cleaner than those from years past, also produce more smog-forming pollution than typical gasoline-powered hybrids.

UCS is working to ensure that automakers reduce emissions from both diesel and gasoline vehicles. To learn more, visit www.ucsusa.org/clean_vehicles. To compare the emissions performance of specific diesel and hybrid models, visit www.fueleconomy.gov.