18.4 cents per gallon is a federal tax. States and local governments add on various sales and per gallon taxes that average close to $0.50 per gallon, though the sales tax portion will have driven some of that tax cost down. California adds 58.8 cents per gallon of taxes plus a 2.25% sales tax. Other states can be much lower but no matter where you are, there's a price floor due to taxes.