This article presents an experimental validation of energy savings achieved through cooperative driving automation (CDA) measured by vehicle-in-the-loop (VIL) testing in car-following scenarios. The impacts of different CDA classes—from status sharing to prescriptive—on vehicle energy efficiency are explored. In the experiments, a plug-in hybrid electric vehicle runs on a chassis dynamometer integrated with simulation software that creates a virtual environment. Results indicate that when agreement-seeking cooperation operates with even a minimal number of vehicles, energy can be saved by up to 5% over human driving. Our findings highlight the considerable promise of CDA technologies for enhancing energy efficiency, especially fostering research on agreement-seeking cooperation.