Underhood Thermal Management has become an important topic for the majority of automotive OEM's. To keep combustion engines cool and manage waste heat efficiently is an important part in the design of vehicles with low fuel consumption. To be able to predict cooling performance and underhood airflow with good precision within a virtual design process, it is of utmost importance to model and simulate the cooling fan efficiently and accurately, and this has turned out to be challenging. Simulating the cooling fan in a vehicle installation involves capturing complex fluid dynamic interaction between rotating blades and stationary objects in the vicinity of the fan. This interaction is a function of fan rotation rate, fan blade profile, upstream and downstream installation components. The flow is usually highly turbulent and small geometry details, like the distance between the blade tip and the fan shroud, have strong impact on the fan performance characteristics. Fan installations therefore have a large influence on cooling performance which the fan data from the supplier cannot capture. Improved simulation capabilities in this area are critical for optimizing the design of energy efficient vehicles since the performance of these fans, which provide airflow to the heat exchangers used for engine cooling and HVAC system operation, have a big impact on the vehicles' overall energy efficiency.This paper presents a comparison of two methodologies for simulating fan air flows. Multiple Reference Frame (MRF) and Sliding Mesh (SM) techniques are applied for a typical heavy duty truck fan. Simulation results are compared to experimental data obtained in a fan test rig with representation of a typical truck fan installation, including fan shroud, ring, seal and an engine silhouette downstream. Results of MRF simulations are known to be sensitive to the MRF domain, which is highly constrained in tight fan installations. For typical truck installations, SM provides a more robust alternative, and better accuracy than MRF in the transitional and radial regime of the fan curve.