Every time you stop to fill your tank, check the gas cap seal for damage and make sure your gas cap is tightened when you finish. Fuel evaporates through a loose cap or a damaged seal, reducing your gas mileage by as much as 2 percent and potentially allowing contaminants and dirt to enter the fuel system. Escaped fuel fumes can also pollute the air, so taking the time to inspect your gas cap can not only save you money over the lifetime of your car, but it can be good for the environment. On all newer vehicles, a leaking gas cap will cause the “Check Engine” light to stay on. If you discover that your gas cap is indeed broken or leaking, you will want to replace it immediately. Your local auto supply store carries gas caps for most vehicles.