Recent AR research efforts have explored the use of virtual environments to test augmented reality (AR) user interfaces. However, it is yet to be seen what effects the visual fidelity of such virtual environments may have on AR interface assessment, and specifically to what degree assessment results observed in a virtual world would apply to the real world. Automotive AR head-up (HUD) interfaces provide a meaningful application area to examine this problem, especially given that immersive, 3D-graphics-based driving simulators are established tools to examine in-vehicle interfaces safely before testing in real vehicles. In this work, we present an argument that adequately assessing AR interfaces requires a suite of different measures, and that such measures should be considered when debating the appropriateness of virtual environments for AR interface assessment. We present a case study that examines how an AR interface presented via HUD effects driver performance and behavior in different virtual and real environments. Twelve participants completed the study measuring driver task performance, eye gaze behavior and situational awareness during AR guided navigation in low- and high-fidelity virtual simulation, and an on-road environment. Our results suggest that the visual fidelity of the environmental in which an AR interface is assessed, could impact some measures of effectiveness. Discussion is guided by a proposed initial assessment classification for AR user interfaces that may serve to guide future discussions on AR interface evaluation, as well as the suitability of virtual environments for AR assessment.