A feature designed to improve fuel economy has been included in the automatic transmission on your vehicle.

A clutch within the torque converter engages automatically at calibrated speeds. This may result in a slightly different feeling or response during normal operation in the upper gears. When the vehicle speed drops or during some accelerations, the clutch automatically disengages.

NOTE:
The torque converter clutch will not engage until the transmission fluid and engine coolant are warm [usually after 1 to 3 miles (1.6 to 4.8 km) of driving].
Because the engine speed is higher when the torque converter clutch is not engaged, it may seem as if the transmission is not shifting into Overdrive when cold.
This is normal. Using the Electronic Range Select (ERS) feature, when the transmission is sufficiently warm, will demonstrate that the transmission is able to shift into and out of Overdrive.