Climatic and environmental conditions have significant effects on pavement performance but have been inadequately considered in previous design methods. The Mechanistic–Empirical Pavement Design Guide (MEPDG) accounts for these effects through the use of the enhanced integrated climatic model. The present study examined the effects of climate on pavement performance predictions and the effects of climatic file generation by using MEPDG. The paper details the effort required to compare performance predictions for composite pavement consisting of asphalt cement over portland cement concrete (PCC) for 610 locations across the United States, with the use of MEPDG, Version 1.0. Analysis of the results was performed at the national, regional, and local levels. Although in general the results agree with the anticipated trends of environmental effects on pavement performance, the prediction of performance according to the predicted transverse cracking in the PCC layer was found to contain inconsistencies. These inconsistencies are attributed to the quality of the climatic data. The paper concludes that the MEPDG climatic database should be cleansed of incomplete or questionable climatic data files to ensure reliable performance prediction.