Nowadays, much focus for vehicle manufacturers is directed towards improving the energy efficiency of their products. The aerodynamic drag constitutes one major part of the total driving resistance for a vehicle travelling at higher speeds. In fact, above approximately 80km/h the aerodynamic drag is the dominating resistance acting on a truck. Hence the importance of reducing this resistance is apparent. Cooling drag is one part of the total aerodynamic drag, which arises from air flowing through the heat exchangers, and the irregular under-hood area.When using Computational Fluid Dynamics (CFD) in the development process it is of great importance to ensure that the methods used are accurately capturing the physics of the flow. This paper deals with comparative studies between CFD and wind-tunnel tests.In this paper, two comparative studies are presented. One is a comparison between cooling performance simulations and chassis dynamometer measurements; the other study is a comparison between external aerodynamics simulations and wind-tunnel measurements.The purpose of this study was to evaluate and develop methods and models for determining aerodynamic drag and cooling performance, which ultimately will be used to develop more energy-efficient cooling of heavy trucks.The results from the two comparative studies showed that there was in general good agreement between CFD and wind-tunnel measurements. For the cooling performance simulations, the analysed parameters were very close to the measured values. For the external aerodynamics simulations, the results were not easy to analyse. The overall results were still satisfactory; for the simulated yaw angles, the drag coefficient (CD) values were less than 4.1% different from measured data.