Hi Guys,

Shade tree (think 4ft pine) mechanic, looking at moving up from my current 34ft bumper pull. Looking at a 5th wheel, wife likes a 38ft Keystone, but I'm concerned about the weight. Dry weight is 10500, and GWVR is 12200. I have a 2011 250 6.2 with air bags (Firestone). I know my GVWR is 12000. According to Ford, the GVWR goes to 15K with the 4.30. I'm really not completely sure why? Is this advisable to do? Just trying to learn and understand beforehand. Thanks!

 

I would say from a legal standpoint your GVWR that was assigned by the manufacturer upon assembly is always and forever the GVWR for that vehicle. Changing the gear ratio may help it tow more easily, but the GVWR on the door sticker is what everybody is going to look at should the need arise.
Pulling a camper that large with an F250 is ill advised. If you really have to have that camper I suggest getting a bit more truck. Your nerves will thank you.

 

 

Yeah, looks like he's pulling max 5th wheel rating or something like that out of the towing manual. Roughly speaking GCWR less a curb weight assumption for the truck.

OP - GCWR is really the point you're hitting at here. Why GCWR increases with lower gears is because of two reason: torque multiplication through the transmission when starting, and gear reduction at speed. Numerically larger (aka lower or deeper) gearing can be thought of as causing higher RPMs at fixed speed.. Having 4.30s instead of 3.73s will increase RPMs at your towing speed letting to engine produce more power at your fixed 65mph point (for example, whatever your speed is). When starting, RPM will be higher at your initial 1-5mph start up, meaning more engine power to be multiplied through TC and a better start. The third consideration is ability to hold speed. As an incline comes up, you could proactively downshift to increase engine output before you start losing speed, but in some marginal cases deeper gearing can prevent a downshift in the first place due to higher output. This can reduce gear hunting in the transmission, but this point is becoming more and more moot as better 6-10 speed transmissions are developed that are much smarter than the old slush boxes.

If you have 3.73 gearing, you'd be better off with 4.30s or deeper at any trailer weight IMO. If you can get the axle weight and loading correct, 12,220# is not outside the realm of what your powertrain can handle with 3.73s, but it's going to come down to your preference on HOW the weight is moved. I hold the opinion of don't ever "fear the gear" and always go as deep as you can with factory gearing in pretty much all cases.