At 60 degrees Fahrenheit, a gallon of pure water weighs 8.34 pounds, whereas a gallon of vehicular gasoline weighs about 6.15 pounds at that temperature. == == Diesel weights about 7.1 pounds per gallon. The answer is really in relation to how much more work the car/vehicle has to do carrying more gas around. Then I suppose you have to offset it with how uneconomic it is to have to regularly stop and fill up. Slowing a car and then accelerating again wastes gas. I suggest removal of all stop signs in the US (I am from the UK by the way)!