A method and apparatus for measuring vehicle exhaust emission includes sampling vehicle exhaust and a dilute mixture of vehicle exhaust and ambient air. A first analyzer measures concentrations of gas components of undiluted vehicle exhaust. A second analyzer measures concentration of at least one exhaust gas component present in the dilute mixture gas component. The at least one dilute mixture gas component is a particular exhaust gas component measured by the first analyzer which is also present in ambient air in substantial quantity. Flow rate of the dilute mixture, the concentration of the at least one dilute mixture gas component and undiluted gas concentration are analyzed by a microprocessor to produce mass emissions of the vehicle exhaust gas components.