Theoretically, the plug-in hybrid can achieve extraordinary fuel-efficiency numbers, given the proper driving schedule. Volkswagen’s proposed XL1 is one such vehicle, reaching 261 mpg, the automaker says. To accomplish that, the XL1 employs just about every imaginable fuel-efficiency technology, including a 47-HP turbo-diesel engine, a 27-HP electric motor, carbon fiber-reinforced plastics in its monocoque, body panels, and anti-roll bars. The brakes also use carbon fiber ceramics, while the engine crankcase, steering gear housing, dampers, and other suspension components are made from aluminum. Even the XL1’s windows were designed with weight reduction in mind, all of them being made from polycarbonate.