[Yake Bait]

I have the same rear diff gear ratuo. Depends on the grade thar you are driving throuh. My truck does ocassionally go into 4th gear on the highway on a steeper grade. If you are going through big hills or pushing through a lot of wind I would think you are OK. On the highway I tow in "D" not "M". I lock out 6th if I a off the expressway or if there are hills.

If you are brand new and towing your shift points could still be a little off.

 

[Yake Bait]

AK has a good point on the wind drag. Noticed today that my truck would not use 6th gear at all while driving 70 mph in crosswind. Slowed down to 60 mph and it would find 6th gear. It seems that my rig really plows at speeds above 65 mph. Mileage really tanks above this speed.

I used my off road screen to see what kind of hill triggers a 4th gear downshift. In general a 2 deg grade will trigger a downshift. . Can shift to 4th on an extended 1 deg grade.