On Ram 3500 Vans the Engine, Tans and passengers are in the front, the weight is not 50% - 50%, so the the 10,720 rating is not on all 4 tires.
When braking the vehicle weight will transfer to the front tires. So the the 10,720 rating is not on all 4 tires. (Front brakes provide 70% of stopping power. ) Failure rate higher braking downhill on warm days.
The quality of the Nexen tires is the other problem. Nexen can not handle the heat of highways and hot days with the front tire load. So I do not think they are 2,680 tires after 10K miles.
The van should have 225/75R16C tires, the C is a Load 121 = 3,195 pounds (4 x 3,195 = 12,780 , more than the GCWR 11,500 lbs) (or other tires with 3,195 rating some R17 have this)
Like:
Goodyear Wrangler Fortitude HT 225/75R16C Load(lbs) 3195
GT Radial Maxmiler Pro 225/75R16C 121/120R E Load Capacity 3197/3086 Pounds
Firestone TRANSFORCE CV COMMERCIAL HT 225/75R16C 121R Load: 3195 pounds
Continental VanContact A/S 225/75R16C E/10PLY Load(lbs) 3195
and more
I would be relatively sure that tire load ratings are done with all the extraneous forces calculated in including braking, cornering, temperature, elevation potholes, and probably a bunch more. I would consider any of that kind of stuff in a tire calculation of acceptable load as it is already done and very complex calculations.
It has been discussed many times that gross combined is a minor factor in tire loading on the van itself when compared to the van weight. You do load the rear tires more from the tongue weight of the trailer, which is typically about 10% (tire loading will be different because of rear overhang with rears gaining and the fronts losing weight) except on large equalization hitch installations. The trailer itself supports the rest of the added weight so not on the tow vehicle tires. The tongue weight is just put into the tow vehicle gross weight calculation and if you measure individual tire loads, the trailer should be on at the time.
That said, getting some extra safety factor in load capacity is certainly a good thing. I am not a fan of running right at max load capacity as it does not allow for any margin or error beyond whatever safety factor the manufacturer put in to cover the unexpected. On our Chevy Roadtrek we upsized the tire size and load rating to give us 18% headspace instead of 8% on the most heavily loaded tire, at our max loading, when in camping mode, which is the left rear. Weights were actually measured at each corner.
I understand the nose heavy of front drive vehicles, but also realize that a large portion of the added weight to an RV conversion winds up on the rear wheels, as does most of the non tank storage are. Rear drive vans are much lighter in the rear when unloaded also, but on our Chevy Roadtrek 190 we are 1000# heavier on the rear. Our van is similar in length to a Promaster at 20' 4" if you don't count the spare carrier.
Hopefully, someone will have had a Promaster on the scales so we can know for sure as it is obviously an important issue.
Personally, I think I would be going with the Euro C rated tires in the case of this thread, over the E rated, unless I was running a worst case tire weight that was well under max load rating for that tire.
