<p>This study experimentally investigates the near-wake dispersion of gaseous pollutants emitted from a light-duty truck operating at two representative speeds (42 and 55 km/h) with diesel and a B50 biodiesel–diesel blend. A 1:20-scale truck model was tested in a controlled wind tunnel to examine the effects of vehicle speed and fuel type on the dispersion of CO, CO₂, NOx, and NO₂. Results show that increasing speed increases vehicle-induced turbulence and accelerates pollutant diffusion, resulting in lower near-field concentrations. At 55 km/h, the diesel–biodiesel blend produces a higher exhaust jet velocity and greater longitudinal dispersion than neat diesel along the vehicle axis, especially in the near-tailpipe region. Utilizing biodiesel reduces CO₂ but increases CO, NOx, and NO₂ emissions due to its higher oxygen content and combustion temperature. These findings demonstrate that fuel composition and vehicle speed jointly determine the vertical and longitudinal dispersion characteristics of exhaust plumes. Beyond the experimental observations, the results provide practical insights for urban air quality monitoring and emission modeling. Specifically, understanding how the biodiesel content of the fuel mixture and vehicle speed alter plume geometry can support optimized placement of roadside sensors and improve the interpretation of remote-sensing data in traffic corridors with mixed biodiesel usage. The presented dataset and analysis thus offer a foundation for refining pollutant dispersion models and developing evidence-based emission management strategies.</p>