This paper examines medium- and heavy-duty truck fuel economy and use characteristics as reported in the 1977 Truck Inventory and Use Survey (TIUS). The study focuses on in-use fuel economy, the use of fuel conservation equipment and annual miles per truck for three vehicle weight categories as functions of engine fuel type and area of operation. The TIUS data show that there is no significant difference between the average fuel economy of gasoline vs. diesel trucks in each of the three weight categories considered in the analysis. Significant differences are shown in annual miles per truck by weight category, fuel type, and area of operation. The data also illustrate a large increase in the use of fuel conservation equipment in both new and older trucks.