How do you determine hitch weight?

MargaretB wrote:
It's surprising to us that a vehicle that's rated to tow 7500# and has a torque of around 440# can only tow a 600# hitchweight.

You're too focussed on that mythical tow capacity number.

When the manufacturer says the vehicle can tow 7500 lbs, they mean that vehicle could tow 7500 lbs if, it is a stripped down model, the driver only weighs 150 lbs, and he has no passengers and no cargo. From that point, everything and everybody put in or on the vehicle, reduces the available payload, and max tow capacity, pound for pound.
If the manufacturers were to rate their max tow capacity, based on all the people and stuff that we campers tend to drag along, they would probably rate the vehicles at half of what they do.

If you have a vehicle that says 7500 lb max tow capacity and put 1000 lbs of people, tongue weight, and cargo in / on it, that tow capacity drops to 6500 lbs.

They also have a rating for what that vehicle can carry. That vehicle can't carry its max payload and pull it's max tow weight, at the same time.
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Example:

Say the vehicle has 1200 lbs payload, 1000 lb hitch rating, and 7500 lb tow rating.

Say the driver weighs 160 lbs, his trailer weighs 7500 lbs, and his WD hitch weighs 100 lbs.

Add up what the vehicle is carrying 160 (driver) + 100 (WD hitch + 940 (average tongue weight) = 1200 lbs.

Theoretically that vehicle could tow 7500 lbs.

Notice there is no cargo and no passengers.
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Read the fine print in most owners manuals. You will find a statement similar to "The vehicles max tow capacity is limited to the weakest link in it's averall ratings".

It's possible to have a vehicle that has a 7500 lb tow rating, with 1200 lb payload, and a hitch receiver that is only rated for 500 lbs. That would limit it's max tow capacity, to approximately 4000 lbs.