If the majority of light-duty vehicles in the United States ran on higher-octane gasoline, the automotive industry as a whole would reduce its carbon dioxide emissions by 35 million tons per year, saving up to $6 billion in fuel costs, according to a new analysis by MIT researchers. In a study published in the journal Environmental Science & Technology, the team considered a scenario in which fuel is manufactured under a redefined octane rating — the measure of a gasoline’s ability to resist engine knocking during combustion.