The drivers behind differences between official and actual vehicle efficiency and CO2 emissions

Literature explaining the gap between official and actual vehicle efficiency and CO2 emissions focuses on descriptive analysis to calculate this gap without examining causality. In this paper, we explore this discrepancy in detail by drawing on a database from Emissions Analytics Ltd. that provides on-road emissions measurement on more than 650 vehicles in the period 2010-2017. The data reveal concerning results: firstly, the gap in data relates both to hybrid vehicles (that are supposedly 'more fuel-efficient') and to the biggest selling vehicles (medium-sized cars). Secondly, the average deviation rate increased prior to 2015, but decreased following 'Dieselgate'. The Volkswagen scandal threw light on the discretionary behaviour of manufacturers on this question and highlighted how weak the official tests are: and this in turn points to a regulatory and compliance problem. In other words, the interpretation of the results suggests that after several years of adaptation to the protocol and the corresponding test (but no translated in real consumption), manufacturers have taken measures to reduce the divergence in real terms after the scandal.