Torque ratio bounds for automatic transmissions

A vehicle includes a multi-speed transmission having an input shaft and an output shaft, an actuator, and a torque converter having an impeller selectively coupled to the actuator and a turbine coupled to the input shaft. A vehicle controller is programmed to, in response to an estimated torque ratio between the impeller and output shaft of the transmission being less than a minimum torque ratio between the impeller and output shaft of the transmission during a shift of the transmission, command torque to the actuator based on a driver-demanded wheel torque and the minimum torque ratio, and, in response to the estimated torque ratio exceeding the minimum torque ratio during the shift of the transmission, command another torque to the actuator based on the driver-demanded wheel torque and the estimated torque ratio.