Fuel efficiency being stated as "miles per gallon" instead of "gallons per mile".

If you take gallons (volume - length x length x length) and divide by miles (distance- length), then you get a number whose units are length x length... or area. So if we used gallons per mile, we'd have a number with easy to understand units.

What would that number with those units represent?
Imagine your car's fuel tank is a very long, thin tube... a miles long tube... The tube holds exactly as much gas as your car needs to travel a distance equal to the tube's length. So a thin tube means low fuel consumption, and a thick tube means high fuel consumption.

Low gallons per mile (gpm) means low consumption, high gpm means high consumption. The units are area, which we all understand. That area is the cross-section of a tube which represents your vehicle's consumption.

It just makes so much more sense.