Major automakers this fall will begin selling cars that produce at least 70% less pollution than vehicles now in showrooms. Under a plan approved by the Environmental Protection Agency, the new models will be sold in 12 states from Virginia to Maine. And, starting with 2001 models, manufacturers will sell the cleaner cars nationwide. Engineers are lowering the emissions in 1999 models by using new catalytic converters, air and fuel controls, and electronic technology. They will place converters in tail pipes as close as possible to the engine. Sensors in the exhaust stream will monitor continually the ratio in the combustion chamber. An optimum ratio of 14.7 parts air for every one part of fuel will be maintained automatically.