Using Soopernaut's VIN tag example:
Rear axle capacity = 3,600
Front axle capacity = 2,500
Total CAPACITY (TC) of truck = 6,100 lbs.*

Now subtract the factory dry weight of the truck say 3,800 for a long bed from the TC. Then subtract another 500 lbs for weight of driver gas, oil and whatnot from the TC.

You are left with a payload capacity of of the truck... how much more weight you can carry. In this case as is typical, about 1,800 lbs. If I were a passenger total payload would reduce by 200lbs and become 1,600 lbs.

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