SYNOPSISA cold-start test procedure has been developed for use on a test track to show the benefits of fuel-efficient engine lubricants. Using this procedure the fuel economy benefits of a friction-modified SAE 15W/40 motor oil over a conventional SAE 20W/50 motor oil have been evaluated in a fleet of eight cars. These cars, which were chosen to be representative of the UK market, ranged in engine size from one to two litres.Although some cars showed large benefits for the lower viscosity friction-modified oil, especially during the first mile from cold start, other cars showed little response to the oil change. Because of the limited number of tests in each car on each oil and the day-to-day variations in ambient conditions, the differences in fuel consumption measured in single cars were not significant at the 95% confidence level. When the eight cars were considered as a single fleet the fuel consumption benefit for the lower viscosity friction-modified oil became highly significant even for the early laps when repeatability is at its worst.The benefits in fleet fuel consumption for the lower viscosity friction-modified oil varied from 5.95% during the first mile from a cold start to 1.94% when fully warmed up after eleven miles.Experience with the test procedure has also shown the importance of using a low volatility fuel for making the fuel consumption measurements.