A method (100) and system (200) for controlling vehicle 1 emissions to achieve a desired emissions conformity factor set by a predetermined threshold value, model or look-up table. The monitored moving average emissions value is compared to the predetermined value in order to control the engine 2 and/or an after-treatment exhaust system 8, maintaining the moving average emissions value at or below predetermined threshold value to achieve the desired conformity factor. One or more sensors 16 measure the average emissions value over a period of time or distance and categorise the measurement in relation to a threshold value, data model or look-up table into two or more categories. The average emissions values are categorised according to driving condition or vehicle wheel power and are related to a quantity of CO2 and NOX emitted. Engine operation is controlled by recirculating a portion of exhaust gases back into the engine inlet 2a and the exhaust gas system is controlled by varying t he amount of reductant available to a selective catalytic reductant (SCR) device 12.
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