With government mandates, original equipment manufacturers are increasingly focusing on fuel economy and finding efficiency gains throughout the vehicle. Lubricant companies have been asked to design fluids that aid in this effort. Demonstrating real gains becomes complex given the intricacies of these systems and methods range from bench top screen tests to component test stands to full vehicle testing. This paper addresses the variation that was encountered when testing automatic transmission fluid efficiency within a full vehicle test. While it is well known that variability in testing conditions such as engine load or vehicle speed will lead to variability in results, the magnitude of their impact on average throughout the test cycle suggests that repeat testing may not be sufficient to guard against improper conclusions. In fact, our data indicates that typical differences in average conditions across the cycle can be associated with predictable impacts in test results of 0.7%-1% for vehicle fuel economy (in miles per gallon) when tested on the Highway Fuel Economy Test (HFET). In this paper we discuss an empirical model that quantifies the impact average operating conditions observed during the HFET test cycle can have on the miles per gallon reported for that cycle. This model explains differences between laboratories as well as replications over time at the same laboratory. In many cases normalizing test results to adjust for the average operating conditions could be an economical way to improve the ability to discriminate among fluids.