Measuring lubricant related fuel economy of internal combustion [IC] engines presents technical challenges, due to the relatively small differences attributable to lubricants. As engine technology progresses, large benefits become harder to find; so the importance of precise measurement increases. Responding to the challenge of meeting CO2 targets, many successful IC engine technologies have been deployed; these include downsizing/rightsizing, mechanical efficiency improvements, advanced charging and combustion systems, thermal management, sophisticated electronic control and calibration. These technologies have been deployed against a back-drop of increasingly stringent emission requirements. Increasing attention is focused on technologies which offer smaller but important contributions. The search for smaller improvements combined with growing engine and vehicle technology complexity increases the challenge of producing high quality data.The aim of this paper is a practical engineering overview of the measurement of engine fuel economy using engine drive cycles.As a global lubricant company, Castrol’s engine experience and customer base is broad, covering the whole spectrum from small handheld power tools to large truck and marine engines. Described here are aspects of our experience in testing passenger car engines, with a focus on detecting the impact of lubricant formulation on fuel economy. A facility delivering critical precise measurements is described, along with the equipment and methodology developed over many years by the engineers who use it.Further, stop:start is investigated, an aspect of “micro-hybrid” technology.Using simulation software, the vehicle load demand was varied to demonstrate the effect of typical vehicle parameters on lubricant differentiation.