Abstract : In this research, we rekindle an old debate by questioning the impact on mitigating policy evaluation of detailing a subsector in a global energy-transportation model. We chose the rening sector because it is a relevant case of a sector for which representation widely diers across models and because it oers a unique set of complex joint production in the energy sector. To investigate whether the level of detail in the description of the renery impacts optimal mitigation options, we take the example of a long-term, national, linear programming based, energy-transport system model (TIMES based). We found that the renery description used in the energy system model matters when trying to evaluate energy or climate policy applied to the transportation sector. It impacts the policy costs but also the technology trajectories chosen at the optimum. Essentially, the balance between energy eciency and carbon intensity of transport may be aected by the accuracy of the description of the pivotal rening sector. Consequently, increasing this sector accuracy level should not only be motivated by the wish to gain wider quantitative insights on potential evolution of the energy system but also by the wish to improve the robustness of the model outcomes.