Agallon of milk weighs eight pounds, so you wouldn't try to carry three or four gallons home on your bicycle. gasoline is a bit less dense than milk—oil floats on water—and 6 pounds per gallon of gasoline is about right. a typical u. s. "car" (whatever it is we drive—averaging somewhere between a prius and a hummer, and including a lot of pickup trucks and minivans) gets about 20 miles per gallon, and drives about 12,000 miles. so, how many gallons of gas per year?