The Mack shift tag for the quadraplex calls it an 18 sp and tells you not to use 5th in Lo or Direct. Back in the late 70's, I had an H-67 with a 9 speed over. It had about a 450 rpm drop between gears. I changed out the duplex auxiliary for a triplex which resulted in a ~300 rpm shift point. Except for the shift from 4th OD to 5th OD which still had the ~450 rpm spread. I believe this was called a 13 speed on the Mack shift tag. My question is how do the 13 sp and 15 sp relate to the Quadraplex? Is it the same transmission as the 13 sp double overdrive with an added lo-lo in the auxiliary? When you are running down the road do you actually use lo-lo as a split? Or is it just a gear you use when in 1st and starting off going up hill or driving off road?

Thanks Mike,

That bring up two more questions,
-what is the rpm range between main box shifts when in lo-lo? It seems to me it would have to be 900 rpm. Seems like a tough shift to make going up a hill at that speed.
-From which lo-lo gear do you shift to lo 1st and increase your speed? Do you actually go from lo-lo 5th to lo 1st?

That lo-lo must be really low.

Pete