Most likely we wouldn't be burning gasoline to compress air for these cars: gas is expensive. Instead, we'd probably use electricity derived from coal, natural

Most likely we wouldn't be burning gasoline to compress air for these cars: gas is expensive. Instead, we'd probably use electricity derived from coal, natural gas, or nuclear, with the possibility of better sources in the future. "Zero-emission" is a reasonable description, as the car itself doesn't give off emissions while running. If you're going to include the emissions given off in the process of producing the fuel, a gasoline-based solution is suddenly much worse, as you need to include the emission costs of finding, extracting, refining, and transporting the gas.