By the end of the century, many U.S. autos will be radically different. Most cars still will be gasoline-powered, but they will share the road with autos that run on alternative fuels like methanol, ethanol and natural gas. Many other vehicles will run on electricity supplied by rechargeable batteries. Vehicle-emission rules adopted by California and several Eastern states assure that thousands of non- polluting electric vehicles will be available in the fall of 1997, with more to follow in subsequent years. Private and government fleets are expected to purchase most of the early electrics. The key question is whether individual motorists will buy them. Some analysts are skeptical, noting the cars' limited range and performance and the inconvenience of recharging.