There are lots of reasons a car's gas mileage can start to go down. Poor maintenance, worn parts and even bad gas can cause your MPGs to drop. But most people don't look at the man in the mirror when they are trying to troubleshoot their gas mileage problem. One of the biggest, if not the biggest contributor to lower gas mileage is driving style. That's right, the way you drive can cause you to burn more gas, quite a bit more in fact. Check out how your driving style can affect your gas mileage.