The regular Toyota Prius is the most efficient car you can buy that doesn't have to be plugged into an electrical outlet. It gets an impressive 44 mpg overall in our testing--with plenty of room for five, a big flexible hatchback, a comfortable ride, and very secure handling. If there's a knock on the Prius, it isn't engaging to drive, feels underpowered and is loud when you ask for max power.
Getting the Prius Plug-in version includes a bigger battery that can be charged from any wall outlet, allowing the car to drive more on electricity and less on gas. A full charge gives an electric range of about 12 miles. While the battery was charged, we measured 67 mpg overall in combined city and highway driving.
When the battery runs out of juice, the car begins running like any other Prius, meaning the gas engine becomes the primary power source. Then its gas mileage drops to 43 mpg, one less than a standard Prius--because it has to haul around the extra weight of the larger and now-depleted battery pack.