Fuel efficiency is simply how well your car uses the fuel it is burning in your engine. Every fuel has a “potential energy”: the potential to affect other objects when provoked. In your car's case, the engine wants to change the fuel's potential energy into enough “kinetic energy” to move a piston, which then turns a crankshaft, which then turns a gear, and so on until the energy is transferred all the way to your wheels. Or, to put it in another way, the fuel's chemical potential energy has been converted into work.
There are two ways in which the fuel efficiency of your car is measured. The one you are probably most familiar with is miles per gallon (MPG), i.e. I can go 27 miles if I have 1 gallon of gas. Of course, in the rest of the world, execpt for Britain and the U.S., the standard is kilometers per liter.
If driving through Canada, you' may notice signs on the gas pumps that will say, “With our gallon, you get more.” Technically, this is true, The Imperial Gallon is bigger than the U.S. Liquid gallon. The Imperial Gallon is 10 pounds of water at 62 degrees Fahrenheit, while the U.S. Liquid gallon weighs in at 8.33 pounds of water at 60 degrees. This figues usually to 231 cubic inches in volume measure.
There is probably a program, somewhere, that will calculate this difference down too the drop. But just knowing you will be getting 1 2/3 more pounds of gasoline is useful enough. And just for odd notes, there is also a U.S. Dry gallon, which is apparently 1/8 of a U.S. Winchester bushel. Yes, that is the information I have been seeking. What a tremendous way to lead off on the definition, sirs. We have been seeking this information. And apparently you can't mix water and grain, as there is no useful reason to convert between these 2 unique American gallons. Because, now, wait for it, a dry gallon is 268 cubic inches. It seems sometimes dictionaries don't want to lead off with the clear answer first.