Higher the grade of gas, better the fuel economy

False. While premium grade is a higher-quality gas, that doesn't mean your car necessarily needs it, according to Consumer Reports. Most cars are designed to run on regular gas, and many cars that recommend premium can also use regular because modern sensor systems automatically adjust to prevent engine knocking, a pinging noise caused by premature fuel ignition. Make sure you read your vehicle's manual before making any changes to the products you use.