Prepare to Plug in for 100-mpg Hybrids.

A new strain of gas/electric hybrid vehicles called plug-in hybrid electric vehicles, or PHEVs, represent the latest wave of fuel-efficiency-boosting technology being explored by carmakers and third-party firms. They promise to boost gas mileage to 100 mpg and beyond by running only on rechargeable electric batteries for limited distances. As the name suggests, they can be plugged in to recharge.