Table 2: Truck Categories in the US

Classes 3 through 5 shown in Table 2 comprise medium-duty trucks, approximately 50% of which are diesel-powered. Approximately 91% of the light-heavy-duty trucks comprising Class 6 are also diesel-powered, as are 99% of the trucks comprising Classes 7 & 8.
Light-duty trucks are typically owned by individuals and driven no more than 20,000 miles a year, and only 4.3% of these vehicles are diesel-powered. In contrast, of the trucks that travel 75,000 miles or more annually, more than 85% are powered by diesel engines.
As we have seen, diesel engines are used predominately in heavy-duty trucks and buses in the US, and the primary reason for this is their fuel economy. At the beginning of 1997, the average diesel fuel price per gallon in the US nationwide was $1.45, which was obviously higher than the price of gasoline. Based on that price, and using the average annual miles traveled and typical fuel economy to calculate the annual fuel cost for heavy-duty diesel vehicles, we get a figure of $11,000, which represents a saving of approximately $5000 yearly over the costs of operating a similar vehicle with a gasoline engine. Thus, in spite of the higher price of diesel fuel, better fuel economy, and lower operating costs are enough to cause the transport industry to rely diesel engines.
Also, if we look at trends in demand for cargo transport in the US, we see a 50% increase in demand during the period from 1960 to 1998, which is clearly related to the growth of the US gross domestic product during that period. It is no exaggeration to say that increased demand for cargo-hauling diesel vehicles is directly related to this growth as well, particularly during the period from 1965 to 1998, during which heightened demand led to an increase of approximately 600,000 in the number of heavy-duty trucks on the road.