3.73 is 4380# while the 3.31 is 4200#. (Hint, use google images to find rural dealers that post white/yellow stickers for all their inventory. Then use their website to look at the stickers of different configurations.) The biggest difference seems to be the integrated trailer brake controller. If you look closely at the chart I posted, the FX4 without the max tow package is limited to 6000lbs. Which is odd because the FX4 has all of the same mechanical* and cooling bits as the Max-tow configured Expeditions, but lacks only the ITB and the pro-trailer backup assist.
*Well, ok, there is still an open question on which springs the FX4 gets both with and without the max tow package...
Very interesting that there are two different spring part numbers, but it's not listed as part of the package. My BIL bought an FX4, and he is under the assumption it was good for 9K but I guess it may not be. He's pulling a 7,500lb camper with it too.. yikes.
Goofy how they do all this right? So an FX for all the hardware minus the springs and is 3K less GCVW, meanwhile still has a 1675lb payload and the higher rated rear axle.. i feel like I'm deciphering some cryptic code.
Finally - wouldn't the fact that the FX4 (3.73s) and a rig with 3.31s have the same max trailer rating really mean that the rear HD springs (
JL1Z-5560-A) be the actual driving factory in GCVW, not the rear axle (albeit 180lbs lower in rating).
On edit - I think that would solidify that the FX4 likely has the softer springs.. a brake controller and a fancy ******** knob can't add 3K GCVW