The impact of the drag coefficient of a vehicle on its fuel consumption is very important. This paper will treat a proposition to reduce the drag coefficient via a reduction of the underhood opening area. The coastdown technique is adopted to find the drag coefficient. Three post-processing methods are then compared. Although, reducing the underhood opening ameliorates the drag coefficient, it influences as well the thermal performance of the cooling system, causing a possible overheating of the engine. For this reason, the impact of the underhood opening area on the cooling air speed is studied in detail as well. The purpose of these tests is to draw some variation laws that govern the response of a vehicle to a reduction in the underhood opening.