The present experimental engine efficiency study explores the effects of intake pressure and temperature, and premixed and global equivalence ratios on gross thermal efficiency (GTE) using the reactivity controlled compression ignition (RCCI) combustion strategy. Experiments were conducted in a heavy-duty single-cylinder engine at constant net load (IMEPn) of 8.45 bar, 1300 rev/min engine speed, with 0% EGR, and a 50% mass fraction burned combustion phasing (CA50) of 0.5°CA ATDC. The engine was port fueled with E85 for the low reactivity fuel and direct injected with 3.5% 2-ethylhexyl nitrate (EHN) doped into 91 anti-knock index (AKI) gasoline for the high-reactivity fuel. The resulting reactivity of the enhanced fuel corresponds to an AKI of approximately 56 and a cetane number of approximately 28.The engine was operated with a wide range of intake pressures and temperatures, and the ratio of low- to high-reactivity fuel was adjusted to maintain a fixed speed-phasing-load condition. This allowed for the investigation of several combinations of intake temperature, intake pressure, and charge stratification at otherwise constant thermodynamic conditions. The results show that sources of engine inefficiency compete as functions of premixed and global equivalence ratios. Losses are minimized through proper balancing of intake pressure and temperature, such that the global equivalence ratio (Φglobal) is as lean as possible without overly lean regions of the stratified charge causing an increase in incomplete combustion. The explored speed-load-phasing combination shows that losses are minimized at conditions where approximately 2/3 of the fuel is fully premixed. The results exhibit a pathway for achieving simultaneous increases in combustion and fuel efficiency through proper fuel reactivity and initial condition management.