Hello Allisonman. I have a 1991 Thomas converted school bus. It has a 190 5.9 rear mechanical engine Cummins with the Allison MT643 Tranny. It has a 4.6 ratio Eaton Axle. Bus runs good and shifts fairly smoothly up stream and a little firm on down shifting coming to stops. Top end speeds is only about 62mph at 2200rpm. Bus looses around 8 to maybe 12 mph on grades. Do you know if it has shift solenoid that should allow it to downshift to compensate power loss or could this be engine issues. I'm lost. I know very little about engine or trans. I have serviced all fuel filters and changed oil and filter and air filter hoping to get more power but had no luck. Can you help?