The velocity through an air filter is a fundamental parameter in the efficiency and pressure drop of the filter. We have measured velocity distributions approaching filters mounted in a range of housings and used an efficiency model to predict efficiencies. The housings have included the universal housing of the SAE J726 Air Cleaner Test Code, a duct similar to those used with the SAE J1669 Passenger Compartment Air Filter Test Code, and models of vehicular housings. The velocity measurements show that velocity approaching the filter is not uniform and the efficiency estimates show that this non-uniformity can have small but significant effects upon the clean filter performance predicted by the efficiency model. The model assumes that the local flow is uniformly distributed over the face of the pleats, and velocity variations within the pleat are not considered. The question of how results following test standards are influenced by velocity field variations is examined. The related questions of whether filter test duct configurations should be rigidly prescribed, and how laboratory test housing and vehicular housing results compare are addressed. The predicted effects of nonuniform velocity fields on measurements of fractional efficiency and gravimetric efficiency are compared.