Results from computational simulations of fuel economy and engine-out emissions are presented for light-duty conventional and hybrid vehicles powered by conventional and high-efficiency combustion engines, including use of port fuel-injected, lean gasoline direct injection, reactivity controlled compression ignition, and conventional diesel combustion. The results indicate that multimode operation with conventional diesel combustion plus reactivity controlled compression ignition, conventional diesel combustion only, and lean gasoline direct injection has the potential to significantly exceed port fuel-injected fuel economy. In all cases, hybridization is predicted to significantly improve fuel economy by permitting the maximum exploitation of high efficiency engine combustion states. Predicted engine-out emissions vary considerably with combustion mode, with reactivity controlled compression ignition generating the highest carbon monoxide and hydrocarbon emissions. On the other hand, reactivity controlled compression ignition is predicted to generate the lowest emissions of nitrogen oxides. Importantly, lean gasoline direct injection and reactivity controlled compression ignition combustion modes are expected to dramatically decrease exhaust temperatures, especially for reactivity controlled compression ignition, which can potentially limit aftertreatment performance. While all results presented are from simulations, the results provide prediction of important details and trends for advanced vehicles that are currently extremely difficult to experimentally study.