The use of ultrasonic gas flow meters has expanded in industrial and household fields owing to their unique advantages. The transit-time method is applied to measure the gas flow rate by determining the absolute time-of-flight (TOF). However, it is inevitable that there is a contradiction between long time scale and high time precision of absolute TOF. In this paper, a novel differential TOF (ΔT) algorithm is proposed for high-precision ultrasonic gas flow measurements. The experimental results proved that ΔT plays a decisive role in the actual flow rate calculation. Therefore, when the absolute TOF is obtained, furthermore, the high-precision ΔT is calculated by the cross-power spectrum method separately. The two echo signals are aligned and compared to obtain ΔT, instead of subtracting the absolute TOFs. Either of the two echo signals is the reference signal of each other, which also avoids the problem of finding reference signals. The results of the calibration experiment indicate that the algorithm improves the stability of zero flow drift under static conditions and the accuracy of the dynamic flow rates. The algorithm allows the adjustment of parameters to be applied to pipes of different diameters. The parameters of the algorithm can be adjusted for application to ultrasonic flow metering systems of different diameters.