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A detailed investigation of aerodynamic car testing in ¾-open-test-sections was made. Herein two main influencing variables (dimensionless length scales) could be identified: first, the relative length of the ¾-open-test-section influences the static pressure gradient along the x-axis and, second, the relative collector area has large effects on the wake of a car. The measured values (i.e. drag) are mainly determined by a combined effect of these two parameters. The basic investigation was made in the Porsche 1:4 model-wind-tunnel with two different types of vehicles {sportscar and van) and in two different scales (1:4, 1:5). The results are graphically summarized. This diagram can predict the differences between full size open-jet-wind-tunnels. These predictions were verified by measuring Porsche production cars and the Porsche calibration car in three other automotive wind-tunnels.