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Reducing transport emissions, in particular CO2 emissions from passenger vehicles, is a key element in mitigating the risk of climate change. Conventional welfare economics recommends the use of comprehensive pricing of carbon emissions, which may not necessarily be the most effective approach in transport systems. This paper uses an evolutionary technology diffusion model to simulate the impact of climate policies on passenger car emissions in the US, UK, Japan, China and India up to 2050, seeking to understand policy interaction. We analyse six commonly seen policy instruments and explore systematically the impact of combining each of these policies by developing 63 scenarios for the US, the UK, China, Japan, and India. We assess both the policies’ effectiveness in achieving emissions reductions and their cost-effectiveness in doing so. We show how the diffusion dynamics of the system can lead to interaction of policy levers, generating synergies in some cases (combined effectiveness more than the sum of its parts), and mutual impediment effects in others (combined effectiveness less than the sum of its parts). The paper identifies particular combinations of regulatory, procurement and fiscal policies that are particularly effective at generating rapid change without needing the use of very high fuel taxes or carbon pricing. Notably, combining electric vehicle mandates with taxes and regulations on combustion vehicles is highly effective, as it simultaneously improves the availability of low-carbon options while penalising high carbon options. Simple principles for policymaking can be inferred.