In the context of an increasing electrification and automation of future mobility, the research and development of efficient powertrains requires enhanced test methods. One important aspect is to consider the complex interactions between a smart driving strategy, the traffic flow, the energy demand and emissions of an individual vehicle as well as the corresponding overall values. Until now, the respective development domains traffic flow control, powertrain control, component design and inter-vehicle communication have usually been considered separately. This article presents a methodology that combines these areas and allows developers to obtain deep and highly realistic system insights, taking into account the mutual interactions between the domains. For this purpose, an X-in-the-Loop validation platform is constructed building upon a vehicular networking, a road traffic, and a vehicle simulator, all coupled with two real drive unit component test benches. We further show how this methodology can be used to investigate the effects of a novel predictive powertrain control algorithm taking into account Vehicle-to-Everything (V2X) communication. Focusing on a typical urban reference route, we demonstrate that our algorithm allows for maintaining the performance by using electric motors with reduced specifications.