One wine measure or US gallon of water weighs 8.34 pounds. An Imperial Gallon is ten pounds of water at 62F and 30 inches of mercury air pressure.
At 60F. The density of water (and any other liquid) is dependent on temperature. So a gallon of boiling water weighs less (8.00 pounds) than a gallon of water at 60F.
So fill up your car first thing in the morning when it should be the coolest, you'll get more pounds of gasoline.
Except that most vehicle fuels are stored underground and are at a pretty constant temperature. Fortunately it's pretty cool down there...
Make it easy to remember; an ounce of water weights, about an ounce. can't be easier than that. So an American pint, 16 ounces, weighs about a pound. And an American gallon, 8 pints, about 8 pounds. Whereas an Imperial pint, 20 ounces; and an Imperial gallon, 160 ounces, about ten pounds. Just like it says, up top.   _{Your Comment}