If one wants to have a meaningful number, the reference should be gallons per TON-mile (or liter/100 ton-mile.

The mpg or gpm gives no indication of what is moved a mile.

40 mpg moving an 85 lb school-child is 40*85/2000 or 1.7 ton-mile/gallon.

5 mpg moving an 80,000 lb load of frozen fish is 5*80,000/2,000 or 200 ton-mile/gallon.

So that semi in front of you is about 200/1.7 = 117.65 times as efficient as your little roller skate.

John