Fuel efficiency and environmental concerns are factors that drive the development of complex solutions for propulsion in heavy working machines. Although these solutions, such as power-split hydromechanical transmissions and hydraulic hybrids, indeed are promising in terms of energy efficiency, they also tend to increase the dependency on accurate, stable control. The realization of this aspect, in turn, relies on continuous testing throughout the development process, usually carried out on expensive, time-consuming prototypes. To lower the development costs and time, hardware-in-the-loop (HWIL) simulations may be introduced as a middle-way between pure prototyping and computer-based simulations. In this concept, some parts of the transmission are represented as hardware while others are included as mathematical models running in real-time in a data acquisition system. This mix of hardware and software allows for high versatility while maintaining a high level of reliability of the results. This paper reports on parts of a study on HWIL simulations of heavy complex hybrid hydromechanical transmissions. Control algorithms for the hardware/model interface in a test rig are derived and their performance are evaluated in HWIL simulations of a mid-sized wheel loader. The results show the importance of fast rig controllers to capture the fast dynamics of the software simulations. It was also found that an important aspect of HWIL simulations is that they are well aligned with their purpose. If so, the simulation yields more reliable knowledge, which is of higher use in the design process of these complex systems. To summarize, HWIL simulations may, if implemented properly, be an important asset in the development of heavy complex hybrid hydromechanical transmissions.