Truck speed loss on grades reduces highway capacity and increses the risk of accidents. The rational design of a truck climbing lane as a solution to this problem requires means for predicting truck speed changes on grades. Experimental measurements of the speed loss of trucks operating on highways were conducted at 20 sites throughout the country. These data were analyzed to compare performance to present guidelines for highway design embodied in the AASHTO Policy on Geometric Design of Highways and Streets. The performance of the straight truck and tractor-trailer population is notably better than reflected in the AASHTO publication. Methods were developed for modeling the hill-climbing performance of the four major truck classes at the 12.5 and 50 percentile population level using empirically determined weight-to-power values. Speed-distance plots are provided for each class on constant grades, along with a simple computer program for calculating speed versus distance on arbitrary grades defined by the user.