Heavy-duty diesel trucks (HDDTs) are a significant source of on-road carbon dioxide (CO2) emissions, primarily because of their low fuel economy and high annual mileage. To design strategies properly to reduce CO2 emissions from HDDTs, a good understanding of their activity and fuel economy is required. The traditional data sources and methods for quantifying HDDT activity and real-world fuel economy have limitations in one way or another. This demonstrated that the onboard electronic control module (ECM) could be another important source of data on HDDT activity and real-world fuel economy. With ECM data from 865 HDDTs, it was found that the total fuel economy was about 6.2 mpg. When adjusted for the fuel used during idle and power take-off (PTO), the driving fuel economy was 6.5 mpg. The mean idle time for this fleet was approximately 33% of the total operating hours, and the mean PTO time was about 12%. The fuel rate during idle averaged 0.6 gal/h, and the fuel rate during PTO averaged 1.3 gal/h. The average speed over the lifetime of this fleet, which included idle, was about 35 mph. When idle was excluded, the average speed of this fleet was about 53 mph. All these values are in the reasonable range.