What’s the most hitch weight

From the standard, section 3.2.1 for Tow-vehicl trailering weight (TVTW)

3.2.1 Tow-vehicles Under 8500 lb GVWR
TVTW is the weight of the tow-vehicle used for EPA emissions and fuel economy certification including all options in
excess of 33% sales penetration plus one 68.0 kg (150 lb) driver and one 68.0 kg (150 lb) front seat passenger, plus the
tow-vehicle manufacturer’s available trailering package and/or any required trailering content (if not included in the 33%
option penetration weight) plus representative aftermarket trailering equipment as specified in 5.2. In the case where a
trailering package is not available from the tow-vehicle manufacturer or it does not include a trailer hitch component, the
representative aftermarket trailering equipment as specified in 5.2 shall include a trailer hitch component.

This is simply specifications for setting up the tests defined in the standard. It does NOTHING to increase the actual load capacities that are stamped on your truck.

 

From later in the standard, section 5.4:

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.

This is the part that people seem to skip over. You still have to be under your GAWR and/or GVWR!! No free lunch.

 

*mic drop*

 

I agree with almost all of this. Heavier tires do take away from your payload. Payload is GVWR minus curb weight. The curb weight of your vehicle includes your tires. Now, I can see the point of saying that they are just "unsprung" and that the suspension is not carrying them, so from that sense it seems that it shouldn't affect your payload. However, it is still more weight that you have to be able to stop and control.

 

You are correct. I am using "payload" and "Vehicle Weight Capacity" interchangeably, and that is incorrect.

Owner Manual defines GVWR this way:

"The (GVWR) is the total weight of the vehicle. When towing a trailer, it is the sum of the vehicle weight (including occupants, cargo and any optional equipment installed on the vehicle) and the tongue weight.

"Vehicle Weight Capacity" is what load Toyota says the truck will carry: tires/wheels would not count against the "Vehicle Weight Capacity" number unless they were in the bed of the truck. Apparent confirmation of that is the warning sticker on my 2017 TSS which indicates "load carrying capacity reduced by....51 kg or 112 lbs". The truck has port added spray-on bedliner, and TSS package which includes 275/55R20 tires/wheels and tube steps. The reduction in "load carrying capacity" seems too low to include anything for the larger 20" tires and wheels. If the reduction does include the tires/wheels, maybe the increase over 255/70R18 and standard SR5 wheels is less than I think.

Bottom line, you can not exceed "Vehicle Weight Capacity", GVWR, GAWR, GCWR, or TWR. If the cargo load causes the GVWR to be exceeded because of tires and wheels you added, then you DO lose "Vehicle Weight Capacity" (your point). Toyota has given very detailed information on towing in the owner manual, and as shown above, multiple conditions will have to be met. At least you won't have to do the calculations but once if you don't significantly change your setup. In spite of detailed tabulation of GCWR, TWR for every model number in the owner manual, GVWR or GAWR is apparently listed only on the doorjamb certification label. And, those ratings will be on the original equipment tires.