A pickup's maximum payload is the weight of all of the items that it carries, including passengers and cargo. Here's how it is determined.
Pickups are separated into classes by their gross vehicle weight rating, or the weight of a vehicle plus everything it carries. Each class has a set weight limit. For example, the Class 3 (F-350, Ram 3500, GMC Sierra 3500) weight limit is 14,000 lbs.
Automakers establish maximum payloads by subtracting the weight of the pickup from the top limit of the vehicle class. For instance, if a Class 3 pickup weighs 8,000 lbs, its maximum payload would be 6,000 lbs.
Ford and GM say they remove the weight of some heavy items -- spare tire (Ford), rear bumper (GM) -- from the base curb weight of their vehicles to set what they call a minimum curb weight. The subtracted weight allows the manufacturer to state a higher payload rating, what they call a maximum payload.
Ram, Nissan and Toyota use the weights of their base pickups to establish maximum payloads, with no standard items removed.