Ford super duty 3.31 rear end. Will it be a big mistake. ?

It would be handy for folks to read the 2021 Ford Super Duty Order Guide when determining axle ratios. The built my truck app on line will show ones choices as you build. Ford is fairly stingy on axle choices depending on model and options. For Instance if one gets 20” SRW tires, only the 3.55 may be selected with a diesel. We tow with a 2012 F-350 diesel 3.31 6R140. When we had a 9400 lb fifth wheel it was just fine. Now that we have upped to 12,700 lb, I wish I had a 3.55 because I drop to fifth going up hills in those states that don’t grade much. As said above, desired axle ratio is dependent on tire size and top transmission ratio. The top ratio of the 10R140 is about 6% lower than the top gear on the 6R140 and the 3.55 is about 7% higher than the 3.31 so I would definitely get the 3.55 given the choice—same engine RPMs at highway speed. That said, if buying off the lot and other things dominate the choice, the 3.31 will just run in lower gears. The transmission is a truck transmission and is built to run in lower gears all day if needed. 9th gear is just 9% lower than 10th.