The use of a detergent-containing fuel additive in a diesel fuel composition, for the purpose of reducing engine power loss and/or reversing a previously incurred power loss, and optionally also for reducing smoke and/or particulate emissions. Also provided is a method for assessing the performance of a candidate diesel fuel composition based on its ability to reduce or reverse power loss, and a diesel fuel composition containing a detergent at an active matter concentration of between 100 and 500 ppmw.
