The EPA says that for every 100 pounds of weight carried around by a car, fuel efficiency drops by around 1%. So if you avoid carrying around 8 gallons of fuel

The EPA says that for every 100 pounds of weight carried around by a car, fuel efficiency drops by around 1%. So if you avoid carrying around 8 gallons of fuel everywhere, you could increase fuel efficiency by around half a percent. Not a lot of savings. https://www.fueleconomy.gov/feg/driveHabits.jsp https://www.fueleconomy.gov/feg/driveHabits.jsp