The objective of this work was to estimate the effects of different filter test housings on measurements of automotive air filter efficiency. This objective was motivated by the questions that arise from designing filters on the basis of uniform face velocities and testing filters in housings providing nonuniform velocities. Velocity distributions upstream of a filter were measured with a laser Doppler anemometer and then used with filtration models to estimate filter initial efficiency. A single filter design was tested in a standard and modified SAE J726 Universal Panel Filter Test Housing and in a housing with a shallow angle diffuser followed by a rectangular duct matched to the filter cross section. The velocity measurements showed that the SAE J726 housing presents a very non-uniform flow to the filter. Housing modifications had small effects. The diffuser-duct housing profiles approach fully-developed duct flow profiles. It is concluded that differences in the velocity distributions provided by different test housings result in small differences in filter initial efficiency that decrease filter testing accuracy and comparability, particularly for very small particles.