It does not matter, the clearance between your truck bumper and your fiver is fairly etched in stone for all rigs. Pin boxes are specified for trailers and not trucks. So when I hook up to your stock trailer or you hook up to mine both our stock bumpers will be in the same location in relationship to the trailer, regardless of truck manufacturer, or rail manufacturer. The detailed installation instructions dictated the location of both rail sets regardless of manufacturer. If you are two inches aft then you are compounding the pin load, if you are two inches forward you have relocated some of the load toward the front wheels as well as relocated the rig closer to the cab, both of these scenarios seem detrimental to me. If your rail kit dictated such an installation then surely it is correct, but anything custom also requires custom calculation based on the trailer length. I believe the 3650 is this figure.
I have to think that the big three truck manufacturers have a specified distance from their axle to the edge of the truck bed or bumper, otherwise we would have to purchase pin boxes to accommodate our trucks rather than the trailer, and goosenecks would have to have an adapter to accommodate inconsistencies. We already know what happens when people try to install a longer pin box to accommodate a short bed truck rather than a slider. (thank God for the 88 degree design)
The 5th wheels and goosenecks being the major market for the 250/2500 and their big brothers in the 350/3500 models SRW and DRW, I'm certain they have taken all of this into consideration It would appear to me that the distance between the centerline of the differential and the edge of the bed or bumper has to be almost identical between models. Any significant difference in distance would naturally smash your garage door or impact your trailer in an "out of camber" turn, climb or descent. Your bumper is where it is in relationship to your trailer, no matter the truck you drive.
It only makes sense that I have a truck that is 19.78 feet in length and a trailer that is 36.5 feet in length, and that my total rig is 56.28 feet overall. Well under the 65 foot max length for Texas. The overall length of the trailer at 39.5 is not important as it has no bearing on the total length of my combination. It could be 45 feet and stick out over the cab of my truck (exaggeration) as long as it does not contribute to the length to the combination.
Like I said earlier, I was told this, I do not know how valid it is, and I make no claim to its validity or consistency, but I do know it is close, and seems like a viable explanation for the elusive meaning of this mystik figure.
After I wrote the above, I decided to go test the idea on my rig which happens to be parked at my house and already hooked up to my truck, I found that the 36.5 is measured from my ladder to the rear edge of my truck bed, which is the exact location that the Curt instructions measured from to locate my rails. So on my rig at least the length of my truck plus the 3650 or 36.5 feet = the total length of my rig going down the highway. And I bet yours is the same.
That measurement cannot work. There are some 6' beds,, there are some 8' beds,, there are some 6'6". Oh,,, and the custom beds...
Some mount the hitch directly over center,, some mount hitch 2" in front of center.
