Downsizing and downspeeding have become accepted strategies to reduce fuel consumption and criteria pollutants for automotive engines. Engine boosting is required to increase specific power density in order to retain acceptable vehicle performance. Single-stage boosting has been sufficient for previous requirements, but as customers and governments mandate lower fuel consumption and reduced emissions, two-stage boosting will be required for downsized and downsped engines in order to maintain performance feel for common class B, C, and D vehicles.A 1.6L-I4 diesel engine model was created, and three different two-stage boosting systems were explored through engine and vehicle level simulation to reflect the industry's current view of the limit of downsizing without degrading combustion efficiency with cylinder volumes below 400 cm₃. Some current engines are already at this size, so downspeeding will become much more important for reducing fuel consumption in the future. Twin-turbocharger, supercharger-turbocharger, and turbocharger-supercharger boosting systems were explored using GT-Power and GT-Drive simulation to demonstrate each boosting system's impact on BSFC and drive cycle fuel economy over the NEDC and ARTEMIS (urban) cycle. Transmission shift points were altered to downspeed each configuration to match equivalent vehicle performance while maintaining the same transmission and final drive ratios to not impact vehicle creep speed and gradeability. The twin sequential turbocharged engine had slightly lower full load BSFC values than the supercharged engines, but this slight penalty was easily overcome through vehicle downspeeding by matching performance of the twin turbo vehicle. Vehicle fuel consumption for the supercharger-based boosting systems was 8-10% lower over the NEDC and 12-14% lower over the ARTEMIS (urban) cycle when compared to a twin sequential turbocharger boosting system.