All of these efforts help protect our world. Reformulated gasoline is a new, cleaner-burning gasoline that helps reduce smog and other air pollution in cities that need the help. Some other cities voluntarily require reformulated gasoline. About 20 percent to 25 percent of all of the gasoline sold in the United States is reformulated. California state law requires that motorists use a different kind of reformulated gasoline to help reduce pollution problems there.
Octane measures a fuel's resistance to engine knock.
That's the metallic pinging sound that can come from an engine that's running on gasoline with too little octane. The right level of octane prevents engine knock. Over time, engine knock can damage pistons and other engine parts. Which level is right for your car depends on several factors, including vehicle age and mileage, driving habits, climate and geography, and individual vehicle characteristics. Price and personal preference also are important.