This paper reviews the literature that compares the fuel efficiencies of bulk commodity transportation modes. Most studies used net-ton-miles per gallon to compare modal fuel efficiencies. Net-tonmiles per gallon have traditionally been estimated from aggregate industry data of total net-tonmiles and total fuel consumed. More recent studies have targeted specific origins, destinations, products hauled, types and sizes of equipment, backhauls, and miles traveled to estimate total fuel consumption. This paper shows that fuel efficiency estimates based only on net-ton-miles per gallon can be erroneous. The paper identifies basic variables and measurement methods that can improve the accuracy of modal fuel efficiency comparisons.