A machine vision vehicle wheel alignment system optical target assembly incorporating an adaptor for attachment of an optical target to a vehicle wheel assembly. The adaptor includes at least three contact points for abutment against surfaces of a vehicle wheel assembly, and an attachment mechanism configured to grip surfaces of a tire mounted to the wheel rim to hold the optical target assembly in contact with the wheel assembly surface. The optical target is secured to the adaptor, and maintained in a fixed relationship to the wheel assembly thereby during a vehicle wheel alignment measurement procedure.