Toyota's Prius line has lost some of its luster in recent years. While it's still synonymous with hybrid cars, its plug-in hybrid model is facing stiff competition that has an edge in terms of pure electric range. The auto giant isn't standing still, though: it's unveiling the Prius Prime as an answer to these pretenders to the throne. The PHEV's electric-only range has only been doubled to a still-modest 22 miles, but it makes up for this with an equivalent efficiency of 120MPGe. If the figure holds up, it makes the Prime the most fuel-efficient plug-in on the market. Toyota chalks it up to the combination of an upgraded hybrid system and a larger battery.