Injector cleanliness is well characterised in the literature [1,2,3,4] as a key factor for maintained engine performance in modern diesel cars. Injector deposits have been shown to reduce injector flow capacity resulting in power loss under full load; however, deposit effects on fuel economy are less well characterised.A study was conducted with the aim of developing an understanding of the impact of diesel injector nozzle deposits on fuel economy. A series of tests were run using a previously published chassis dynamometer test method. The test method was designed to evaluate injector deposit effects on performance under driving conditions more representative of real world driving than the high intensity test cycle of the industry standard, CEC DW10B engine test, . The efficacy of different additive levels in maintaining injector cleanliness and therefore power and fuel economy was compared in a light duty Euro 5 certified vehicle.Full load power loss in low or non-additised fuels was observed (~3%), whereas a keep-clean dose of a deposit control additive (DCA) fully maintained performance. Furthermore, a statistically robust link between injector fouling and an increase in specific fuel consumption (~0.7%) at steady state was established.In addition, mechanisms were proposed for the link between injector fouling and changes in fuel consumption derived from in-cylinder measurements throughout the test duration, as well as exploration of how fuel consumption effects change across the operating range outside of the test cycle.