The first measurements of oil film thickness in the main bearings of an engine in an operating vehicle are reported. Experiments have been conducted during 0-65 mph vehicle acceleration on a test track. With some commercial oils, upon initial vehicle acceleration, oil films on the order of 0.01-0.1 microns occur. As the acceleration progresses the thickness of the minimum oil film occurring in each engine cycle increases and reaches a plateau beyond the first transmission shift. Under identical conditions the shape and magnitude of the minimum oil film thickness traces are different for different main engine bearings.