Plug-in hybrids may one day constitute a majority of the cars on U.S. roads. Like today's hybrids, they have both a gasoline engine and an electric-drive motor whose batteries can be recharged by the engine. But they can also be recharged at a standard wall socket. Given that they'll raise electricity demand and increase power-plant emissions, will they really reduce overall ­greenhouse-gas production? It turns out that plug-ins always result in lower emissions than conventional cars do, and they beat regular hybrids handily--except when the electricity comes from coal (the source of 43 percent of U.S. electricity), according to a study. But as gasoline comes from dirtier oil sources, such as tar sands, plug-ins may win even when powered indirectly by coal, one study author says.

Infographic by Tommy McCall