What speed is shift to final gear

I have a '03 Ram Quad Cab with the 4.7 liter engine and an Automatic transmission. I noticed something lately that seems wrong. At 50 MPH, the Tach indicates a little less than 1500 RPM. At 40 MPH it indicates over
1500 and at 45 MPH it indicates 1600 RPM.
It seems to be shifting to the final gear around 50 MPH, but I thought that in the past it made this final shift at around 35-40 MPH. Can anyone tell me what their truck does under these speeds? I am not sure of the transmission, but it is the standard one for that engine.
Don
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Whether the torque converter is locked up or not could be the cause of the variation you describe.

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