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A fuel-based approach is used to estimate long-term trends (1990–2010) in carbon monoxide (CO) emissions from motor vehicles. Non-methane hydrocarbons (NMHC) are estimated using ambient NMHC/CO ratios after controlling for nonvehicular sources. Despite increases in fuel use of ∼10–40%, CO running exhaust emissions from on-road vehicles decreased by ∼80–90% in Los Angeles, Houston, and New York City, between 1990 and 2010. The ratio of NMHC/CO was found to be 0.24 ± 0.04 mol C/mol CO over time in Los Angeles, indicating that both pollutants decreased at a similar rate and were improved by similar emission controls, whereas on-road data from other cities suggest rates of reduction in NMHC versus CO emissions may differ somewhat. Emission ratios of CO/NO_x (nitrogen oxides = NO + NO₂) and NMHC/NO_x decreased by a factor of ∼4 between 1990 and 2007 due to changes in the …