3:55 or 4:10

I've been told that with the new 6 speed transmissions that gear ratio is not as important as in the past. That said I think the 4:10 is best for towing (that's what I have) but if given a choice I would go with 3:73. Its right in the middle.

Diesel or gas ? 3.55 is pretty tall for that heavy of a FW either way in my opinion.

4.10 have 15 percent greater torque multiplication than 3.55. Which one is better for you depends on the engine, tire size, and what speed you intend to drive.

It depends on tire size engine torque/power curves, transmission gear ratios, your specific use....weight/desired towing speeds/hills. Our Peter built has 3.43 final drives and tows over 100,000 lbs.

Based on the scant information given, between the two ratios, I would go with the 4:10 to pull a 15000 lb FW.

Gas mileage will suffer a little when driving solo, but you will appreciate the towing quality much better. It's always a compromise.

With fords 6 speed transmission and the power of the 6.7 Youll probably notice very little in performance

With the truck in your sig or other new diesels, 3.55 is more than sufficient for a 15K fiver. With a gas truck, especially an older one, 4.10 may not be low enough. There are many more important factors besides rear axle ratio.

I do wish Eaton made a 2 speed rear end for pickup trucks..

I used to drive a White with a 5 speed transmission and a 2 speed rear end.
carrying 25,000 pounds of LEAD...almost daily

Tugging 15k is doable all day with the newer diesels and 800+ ft lbs on tap, however if it was primarily a tow vehicle I'd opt for the low gearing given the option. Double OD makes up for low gears. In fact I think 6th is too over driven in the 3.42 and 3.55 geared trucks.
If your talking about gassers, get 4.30s and keep wishing you had more with that kind of weight.

I TALK ABOUT A 2017 FORD DRW DIESEL