In the United Kingdom, passenger cars are the largest contributor to green house gas (Carbon dioxide (CO2)) emissions from road transport. Estimates of CO2 emissions (and associated fuel consumption) are mainly based on data obtained from the rolling road testing of new car models as managed by the Vehicle Certification Agency. The main outputs of this testing for each car model are the expected CO2 emissions in g/km and the expected overall miles per gallon achievable. There are three main areas of concern addressed in this article – that the car emission testing procedures do not accurately reflect outputs achievable under normal on‐the‐road driving conditions, that the test outputs are used directly by government in building green house gas inventories and that the reported test outputs for individual car models are misleading in relation to car purchasing decisions. The article in particular challenges the current UK fleet estimates reported by the Department for Energy and Climate Change that, it is argued, are likely to be significant underestimates. The main aims have been to model the required adjustments of emission factors derived from rolling road tests in order to account for on‐the‐road effects. Results are compared and contrasted with official governmental estimates.
