smokeywren

Not much difference unless are overloaded. The more you're overloaded the more you need the shorter legs (more leverage) of the 3.73 ratio.

3.73 minus 3.55 = 0.18, divided by 3.73 = 0.048 = almost 5%. That means 5% less engine RPM with 3.55 ratio. So 1,700 RPM with 3.55 would be 1,785 RPM with 3.73. Depending on the power curve of your engine, that extra 85 RPM could be a lot of extra horses for dragging a heavy trailer over the mountain.

In my case I have the 3.15 ratio with the EcoBoost. My tow rating is 8,400, but I can easily pull a trailer grossing the 11,300 pounds tow rating of an F-150 EcoBoost with 3.73 axle ratio and HD Payload Pkg.. I wish I had the 3.73 when towing heavy through the Hill Country northwest of Austin, but that's not because of a lack of power, it's because the EcoBoost engine sucks gas when both turbos are working hard. On one trip through the Hill Country when severely overloaded, I averaged about 6 MPG. On that same trip with that same load, I would expect closer to 10 MPG if I had the 3.73 ratio. And normal towing of my 5,000-pound TT, I get about 10 MPG with my 3.15 axle, and I would expect about 12 MPG with the 3.73.

I ordered the 3.15 ratio on purpose, because I expected better unloaded MPG with the 3.15 ratio. But that's not working out because you get good MPH only at slow speeds around 55 MPH when the twin turbos are not working. But I usually cruise at 79 MPH, so both turbos are working and sucking gas, so I average only 12 to 14 MPH unloaded at 79 MPH.

Beginning with the 2015 models, the Heavy Duty Payload package can be ordered with ordinary 18" wheels and tires and includes the 3.73 e-locker axle. So if I were ordering one today, that's what I'd order.

Velosprout

I agree and ordered the HD Payload package 3.5 Ecoboost with the 18" wheels and the e-locker in March. Like others, my order was cancelled by Ford in June. Anyone who wants one today will have to wait and order a '16.