Since Nissan XD diesel was mentioned...
Nissan is way over-hyping this truck. Although it comes with the gooseneck hitch, one may be able to tow a double horse gooseneck trailer without exceeding the GVWR and GAWR with one or two horses inside. I don’t know how they come up with the unrealistic 12,300 towing capacity for gooseneck/5th wheel trailers when the max possible payload is only 2,091, and this particular style is not recommended for towing gooseneck trailers, etc. The Titan XD is only SAE J2807 compliant with the Class 4 Hitch.
Details here.
The J2807 standard has a loop hole built into it that allows the lighter trucks to be over GVWR but not GAWR to achieve GCWR numbers. I have copied two sections that speak to this from the standard and also had a followup email from Toyota stating that the Tundra DC is setup for towing a fifth wheel and consideration to weight should be in this order. GCWR, GAWR, and payload. Section 5.4 points this out and the words and or is used, "or" allows them to be over GVWR but not RGAWR. Now you now why the axle rating is higher than GVWR and why they say you can tow more than what the GVWR will allow. The same goes for 2500/250 and 3500/350 as well this is not just for "1/2 tons". Why not just raise the GVWR for half tons you ask? Two reasons, EPA and CAFE, they look at GVWR not GAWR to classify trucks.
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4.4.3 Tow-vehicle Understeer Specific Ballast Conditions
Ideally, the tow-vehicle/trailer combination should be ballasted such that the tow-vehicle simultaneously attains GCWR,
GVWR and Rear GAWR. If testing with a weight distributing hitch, ballast distribution shall occur prior to applying the
weight distributing hitch torque. The weight distributing hitch shall be adjusted to provide FALR levels within ±10% of
target FALR (e.g. from 40% to 60% for a 50% FALR target) as specified in 4.4.1. Refer to Appendices B and C for
procedure. There may be tow-vehicles that cannot attain GCWR, GVWR and Rear GAWR simultaneously with tongue
weight at 10% (conventional trailer) or kingpin weight at 20% (fifth wheel or gooseneck trailer) of loaded test trailer weight;
in this case, priority in meeting these values should be: 1) GCWR, 2) RGAWR and 3) GVWR. Again, refer to Appendices
B and C for details.
5.4 GVWR/Rear GAWR and Tongue Weight/Kingpin Weight Considerations
The tow-vehicle shall be able to accommodate appropriate trailer tongue and/or kingpin weight to attain a particular TWR
without exceeding Rear GAWR and/or GVWR. Required minimum conventional trailer tongue weight shall be 10% of
TWR and required minimum fifth wheel or gooseneck trailer kingpin weight shall be 20% of TWR. For purposes of this
standard, fifth wheel or gooseneck trailer kingpin weight shall be applied directly over rear axle centerline unless a
fixed-ball hitch is available from the tow-vehicle original equipment manufacturer (OEM) in which case it shall be at the
OEM position. Conventional trailer tongue weight shall be applied at a longitudinal connection point as indicated in 5.5.
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