Passenger vehicles are a major source of greenhouse gas emissions and prodigious consumers of petroleum, making their fuel economy an important focus of energy policy. Whether or not the market for fuel economy functions efficiently has important implications for both the type and intensity of energy and environmental policies for motor vehicles. There are undoubtedly imperfections in the market for fuel economy but their consequences are difficult to quantify. The evidence from econometric studies, mostly from the US, is reviewed and shown to vary widely, providing evidence for both significant under- and over-valuation and everything in between. Market research is scarce, but indicates that the rational economic model, in general, does not appear to be used by consumers when comparing the fuel economy of new vehicles. Some recent studies have stressed the role of uncertainty and risk or loss aversion in consumers’ decision-making. Uncertainty plus loss aversion appears to be a reasonable theoretical model of consumers’ evaluation of fuel economy, with profound implications for manufacturers’ technology and design decisions. The theory implies that markets will substantially undervalue fuel economy relative to its expected present value. It also has potentially important implications for the welfare analysis of alternative policy instruments.