I talked to a to a mack tranny mechanic at the mack dealership and was told the 13 and 18 speed mack trannys are exactly the same, there is no difference in the gears. The only thing difference is the knobs. The 13 speed knob locks out the low side, the 18 speed knob lets you shift the low side. So if you have a 13 speed mack tranny, you can either buy a 18 speed knob and install it, and i think the cost of the knob is around 300 dollars, or take the the 13 speed knob apart, break off or cut out the lock out tang and you have yourself an 18 speed. So if your're ordering a new mack truck, order it with a 13 speed tranny, change or modify the shift knob yourself and you can save yourself the price between a 13 speed and an 18 speed transmission.
this is true. i did this years ago when I had a 13spd mack trans. Take the knob apart and you will see the tang to remove. easy job. ran it for years ...no problems.
