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Wind tunnel force-balance and wake-traverse tests were made on .154-scale car models with various degrees of streamlining to determine the significance of ground treatment for increasing levels of aerodynamic cleanness. The wake-traverse analysis included investigations of spanwise distributions of vortex and viscous drag, which gave insight into the flow mechanisms involved. It was found from these tests that thick, uncontrolled tunnel-floor boundary layers yielded wakes with viscous “side-lobes” at floor level, which were absent with a moving ground representation. For “bluff car designs, this was the only effect. For “slippery” designs, more typical of modern design practice, other more significant changes were noted. Measured changes in trailing-vortex strength and the associated vortex drag suggested that very low-drag designs experience an increase in effective angle-of-attack when the ground is fixed.