Assuming you're getting good acceleration in both conditions, and you are not getting a check engine or flashing OD light, you should have nothing to worry about.
At 50, you're likely just kickin' down to the top of 2nd gear - 2 gears down from OD. At 65, you're kickin' down to 3rd most likely. As long as you're not into the redline of the tack (if you have one) don't worry about it.
