Injector cleanliness is well characterised in the literature as a key factor for maintained engine performance in modern diesel cars. Injector deposits have been shown to a reduce injector flow capacity and result in loss of full load power; however, deposit effects on fuel economy are less well characterised. A study was conducted with the aim of building understanding of diesel injector nozzle deposits on fuel economy. A series of tests have been 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. A light duty Euro 5 certified vehicle was tested and performance degradation was compared with fuels with a keep-clean dose of a new fuel borne additive versus additive free and low additised fuels. Full load power loss in the low or non-additised fuels was similar to previous studies (full load power change, x ̅ = -2.82%), compared to a keep-clean dose of the new additive (full load power change, x ̅ = -0.04%). Further, we have demonstrated a statistically robust link between injector fouling and an increase in specific fuel consumption at steady state (ΔBSFC = +0.63%; R = 0.05). In addition, mechanisms are proposed for the link between injector fouling and changes in fuel economy derived from in-cylinder measurements throughout the test duration, as well as exploration of how fuel economy effects change across the operating range outside of the test cycle.