Research on dual-fuel engine systems is regaining interest as advances in combustion reveal enabling features for attaining high efficiencies. Although this movement is manifested by development of advanced modes of combustion (e.g., reactivity controlled compression ignition combustion, or RCCI), the possibility of gasoline / diesel conventional combustion exists, which is characterized by premixed gasoline and direct-injected diesel fuel at conventional diesel injection timing. This study evaluates the effects of operating parameter on fuel conversion efficiency for gasoline / diesel conventional combustion in a medium duty diesel engine. Through adjustment of gasoline ratio (mass basis), injection timing and rail pressure (with adjustments to diesel fuel quantity to hold torque constant), the combustion, performance and emissions are studied. The results show generally decreasing brake fuel conversion efficiency as gasoline ratio increases, by between no change to 1.4% (relative to pure diesel fuel operation) at medium and high loads with gasoline ratio increasing from 0 to 0.5 and 0 to 0.33 respectively, and by between 2.0% to 4.0% (relative to pure diesel fuel operation) at low load with gasoline ratio increasing from 0 to 0.5. Analysis focuses on the effects combustion, work, exhaust, and heat transfer losses have on the changes to brake fuel conversion efficiency. At dual-fuel operation high combustion and heat transfer losses are mainly responsible for low fuel conversion efficiency, while low exhaust and friction losses are beneficial for fuel conversion efficiency. At low load, the fraction of combustion loss in the fuel energy delivery rate (i.e., combustion inefficiency) increases from 0.7% at pure diesel fuel operation to 8.7% with 0.27 gasoline ratio and to 15.9% with 0.5 gasoline ratio; these increases are attributed to overly lean mixture and prolonged combustion. At medium and high loads, the higher temperature and pressure in dual-fuel operation can explain the higher heat transfer loss.