PITTSBURGH—Will that hybrid vehicle pay for itself and help the environment? That depends on how and where you drive, Carnegie Mellon University researchers report.

Jeremy Michalek, a professor of mechanical engineering and engineering and public policy at CMU, and Orkun Karabasoglu, a mechanical engineering research assistant, analyzed the potential cost and greenhouse gas savings of hybrid and electric vehicles under different driving conditions.

"We found that for highway drivers, hybrid and plug-in vehicles cost more without much benefit to the environment," Michalek said. "But for drivers who experience a lot of idling and stop-and-go traffic, a hybrid could lower lifetime costs by 20 percent and cut greenhouse gas emissions in half."

The study, funded by the National Science Foundation, Ford and Toyota, appears in the journal Energy Policy as the EPA is rolling out new fuel economy labels starting with 2013 vehicles.

Whether or not a hybrid vehicle will pay for itself and help the environment depends on how and where you drive, Carnegie Mellon University researchers report. Photo courtesy of Carnegie Mellon University

"The new labels are improved, but no single test can capture all kinds of driving," Michalek said. "Hybrid and plug-in vehicles will do the most good at the lowest cost if adopted by drivers who spend a lot of time in traffic. For these drivers, hybrids are a win-win, and the benefits may be much more than the labels suggest."

The U.S. government uses standard laboratory tests to measure vehicle fuel efficiency for federal fuel economy labels and standards.

"The fuel economy standards are still based on old lab tests that make vehicles appear to be more efficient than they really are," Michalek said. "This has always been an issue, but it is amplified with today's vehicle technologies. These tests may be underestimating the relative real-world benefits of hybrid and plug-in vehicles."

Driving conditions affect not only cost and emissions, according to Michalek. "Aggressive driving can cut vehicle range by 40 percent or more. That's a notable risk for pure electric vehicles, which already have limited range and take a long time to recharge. But with hybrid electric vehicles, which run on gasoline, and plug-in hybrid electric vehicles that use electricity for short trips and switch to gasoline for longer trips, there's no added risk of being stranded," he said.

Michalek reports that "the bottom line is: before you buy, consider how you drive."