It’s not overdrive that will kill a transmission, it’s long hard pulls with an unlocked torque converter
And when it is unlocked , the 6th or 5th gear is essentially overdrive.
5th and 6th gear are always OD, whether or not the TQ converter is locked. 6 speed Allison transmission gear ratios are as follows:
1st. 3.094:1
2nd 1.809:1
3rd 1.406:1
4th 1.000:1
5th .711:1
6th .614:1
OD isn't some "system" outside of the normal transmission, it's just any gear that's higher than 1:1.
"Tow/Haul" mode doesn't "lock-out" OD. Tow/Haul adjusts shift points to typically hold more RPM both shifting up and down. Tow/Haul will generally result in a "stiffer" or "harder" shift as well by reducing the amount of slip allowed. Tow/Haul doesn't lock out any specific gear and really has nothing to do with OD, OD is simply a gear/gear-ratio.
