A road test has been conducted to quantify the benefits provided by two additive packages in a fleet of modern cars from the European market. One of the packages contains a high-performance detergent, the other contains the same detergent at lower concentration and a spark-aiding constituent. The test was designed to enable a critical three-way comparison to be made between the base fuel and the same fuel treated with the additives. Dynamometer measurements were made according to both the existing ECE Regulation 15 and the proposed Extended ECE test procedures.Through careful test design and execution, it has been possible to assess the performance of the additives at a high statistical confidence level. Both provide a consistently high level of inlet system cleanliness which leads to substantial reductions in fuel consumption and exhaust HC and CO emissions. NOx increases slightly, but the sum of HC+NOx (controlled in ECE legislation) does not.