This may confirm what you question.
If it is a 3060 it should shift to 6th at over 55MPH when you ease off the throttle.
This chap is an Allison man in Portland, OR. He has a thread here that anwers many questions you might have.
"LOVE and LOSS, are two of the greatest emotions one can experience. -- I went to school to learn about "WHAT GOES UP MUST COME DOWN" but I had to live my life to learn the lesson of: 'WITH LOVE THERE WILL BE SORROW'."
David Stewart. (after loosing my NAVIGATOR)