Current SAE practices for evaluating potential improvements in fuel economy on heavy-duty vehicles rely on gravimetric measurements of fuel tanks. However, the recent evolution of portable emissions measurement systems (PEMS) offers an alternative means of evaluating real-world fuel economy that may be faster and more cost effective. This paper provides a direct comparison of these two methods based on a recent EPA study conducted at Southwest Research Institute. More than 228 on-road tests were performed on two pairs of class 8 tractor-trailers according to SAE test procedure J1321 in an assessment of various chassis components designed to reduce drag losses on the vehicle. During these tests, SEMTECH-D™ portable emissions measurement systems from Sensor's, Incorporated were operating in each of the vehicles to evaluate emissions and to provide a redundant measure of fuel economy. These measurements showed excellent correlation to the gravimetric results with a coefficient of determination greater than 0.98 and nearly identical regression slopes for three of the four trucks. One truck had a series of suspect data toward the end of the study that biased the regression slope higher by 4%. Measurement variability also compared favorably between the two test methods. The average coefficient of variation based on the three repeat laps performed on every test segment was 2.98% for the gravimetric measurements and 3.26% for the SEMTECH-D measurements at a 95% confidence interval. These results all support the use of SEMTECH-D as a viable alternative to the gravimetric measurements for heavy-duty in-use fuel economy determination.