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The accurate confirmation of fuel economy benefits arising from the use of a gasoline additive package is a difficult exercise. In order to fully understand, and accurately quantify, the interaction between the treated fuel and the engine it is necessary to carry out a fleet test which allows the benefit to be measured precisely yet under realistic conditions. A series of road tests, based on a ‘paired fleet’ design, has been carried out over the last two decades. Careful analysis of the results of these tests has led to the design of a new fleet testing protocol based around ‘quads’ rather than pairs of vehicles. This new test protocol also incorporates advances in vehicle selection and preparation procedures and has resulted in a test that is both more robust and flexible, and that achieves even better resolution than before.

It is desirable to predict the cold weather driveability performance of fuels by means of an index based on simple measurements such as the ASTM distillation curve. In the past, several such indices have been proposed from the analysis of vehicle test results. In contrast, this paper describes how a driveability index can be derived from first principles - namely, the physics of fuel vapour formation. A number of present and proposed driveability indices were evaluated by comparing the way they correlate with the calculated enthalpy requirement of fuels. It is concluded that E100+E150 best meets the need for a simple index and is robust across a range of air/fuel ratios.