Most of the newer Allison transmissions have the dynamic shift scheduling/economy mode on and it does screw with the shift patterns. Although the majority of our buses are capped at 40 MPH, the normal shift pattern does not go into 6th gear, instead staying at 5th. At 40 MPH in 6th gear it makes the bus a little harder to stop, plus any inclines will screw with the acceleration when the vehicle does lose speed, so it's best to keep in 4/5th gear to have a little more control. Hell, I do it with our C40s, I usually limit them to 5th gear on dry pavement and drop it to 3/4th in worsening conditions