Acoustic velocimeters for fluids traditionally assume a straight ray between transmitter and receiver, and use acoustic phase velocity for calculating fluid speed or velocity from upstream and downstream transit times. Corrected ray theory which uses group or signal velocity yields a relation between fluid velocity and differential transit time which differs from the traditional expressions. Fluid velocities calculated from differential transit times in the range 0.1–50 μs over a short path (5 cm) in air differ by as much as 1.4% using the traditional and corrected ray methods.