Shift! **** You Shift!

RoyJ wrote:

swimmer_spe wrote:
But the lighter trailer, pulled faster would not have shifted up. It would have stayed in 5th.

Have you tried turning off cruise?

I almost never tow / haul with cruise on, because I want to know exactly what my throttle position is.

You may find with the cruise off, you are able to pull in 6th, by keeping the throttle position just under the point where it triggers a downshift. If there's a slight hill or headwind, as long as you hold your throttle steady, the truck would bleed off a bit of speed, and you may be still locked in 6th.

With cruise, depending on the programming algorithm, the slightest speed loss may cause the computer to over-compensate and grab 5th.

If you can't even get into 6th, try accelerating to say, 115, then back off the throttle to let it upshift, then maintain enough throttle to hold 110. If it can't, then you know your engine simply don't have the power at that rpm to do it. Could be a lighter trailer has higher wind drag.