The efficiency of an automotive internal combustion (I.C.) engine is dependant on many factors such as combustion efficiency, thermal transfer efficiency, and mechanical efficiency. In this paper, the mechanical efficiencies of a fluid film journal bearing (crankpin) and a roller bearing operating under identical conditions are calculated. Comparisons between the two bearing types for various load and speed conditions are presented. Plots of heat generation, coefficient of friction, and oil film thickness versus speed are used to compare the two bearing types. The analysis indicates that frictional losses and heat generation may be reduced significantly if roller bearings were used in place of journal bearings in I.C. engine crankshaft assemblies.