It has long been asserted that optimizing injector cleanliness (preventing the formation of deposits and removing them if they are present) can have a positive impact on fuel economy for heavy-duty diesel engines. However limited amounts of field data exist on more recent engines to support these claims. Also, factors such as small test sample size, varying BTU content of fuel and inconsistent duty cycles in test vehicles can often skew fuel economy study results in a field-test environment. This paper will detail an extensive field study (in which care was taken to minimize the effects of the above mentioned factors that can introduce error) to analyze the effect of consistent dispersant/detergent usage on fuel economy for a grocery delivery fleet. Thorough statistical analysis of data for a total of 24 units will show a 0.53 % fuel economy increase (calculated from the average lower bound mean difference for each unit with a 95% confidence limit) when the dispersant is used consistently.