Abstract: Automotive wind tunnels are essential in replicating the real-road aerodynamicenvironment for vehicle testing. However, the confined dimensions of the 3/4-open test section, along withthe ground boundary layer and jet shear layers, introduce disturbances to the airflow, leadingto errors in aerodynamic drag measurements. Utilizing a simplified potential flow model, thisstudy identifies and quantifies four critical systematic errors within the test section: solidblockage, nozzle blockage, collector blockage, and horizontal pressure gradient. The solidblockage tends to underestimate the measured aerodynamic drag, while the nozzle andcollector blockages result in overestimations. The interplay of these effects creates a selfcorrecting mechanism that enables the testing of larger vehicle models within the wind tunnel. Employing the Mercker-Wiedemann correction method at Tongji University's full-scale aeroacoustic wind tunnel, this research conducted a correction analysis on three vehicles of varying sizes. The findings indicate that, with a criterion of keeping the aerodynamic drag coefficient error within 1%, the wind tunnel is capable of accommodating vehicles with ablockage ratio of up to 15%. Moreover, the horizontal pressure gradient is identified as asignificant source of error in the drag coefficient measurement. These insights underscore thenecessity for stringent design and acceptance protocols in wind tunnel engineering with dC_p(x)/dx0.001/m to ensure measurement precision.