The purpose of the study is to model scenarios of technological innovations in the global passenger vehicle fleet, i.e., improvements in the energy economy of average regional vehicle fleets and blending of alternative fuels. This is to quantify the potential CO2 emission reductions that may stem from enhancing "business-as-usual technologies" in cars with respect to a set of baseline car stock projections. The study adopts an international approach quantifying in total 11 world regions, thereby conceptualising regionally distinct growth patterns of average car stocks until 2050. Scenario analysis is used to analyse impacts of alternative futures in car technology, i.e., the adoption of efficiency improvements or the blending of low-carbon biofuels to overcome business-as-usual growth in car-related CO2 emissions. To facilitate the assessment the present study is based on a multi-model approach to car demand, applying two types of methodologies rooted in the economics of consumption, utility maximisation and single equation models, to derived reference scenarios of car stock growth. They assume that preferences are the same throughout world regions, following the American lifestyle of individual passenger vehicle demand. The models are calibrated using empirical data that have been originally collated from international sources for the purpose of the study. Computation results show that given substantial growth in regional vehicle fleets under business-as-usual assumptions particularly in transition and developing regions, technological improvements in vehicle efficiency must be complemented by growing biofuel use with increasing mitigation potential in order to brake the trend of ever rising CO2 emissions. We conclude that a necessary absolute reduction in emissions from the passenger car sector needs tremendous efficiency improvements in the passenger vehicle fleet accompanied by a growing share of biofuel use. However, lifestyle and behavioural changes in overall mobility patterns are imperative to mitigate emissions from the car sector.