I've been wondering about this for quite a while and nobody seems to have an answer. I drive a '97 Ford F-150 pickup and it supposedly is a half-ton truck. What exactly is the half ton we're referring to and what exactly does the 150 stand for?
Actually, the terminology of half, three quarters and one-ton vehicle goes way back. I believe the ancient Egyptians used it originally to specify how much grain a camel would carry before his legs buckled.
In the automotive world it's simply a crude description for "standard vehicle payload"; like saying a half-ton truck has a 1000lb payload capacity. Payload being the amount of passenger and cargo weight the truck may carry safely. Now-a-days you see model numbers like the F-150 on your new Ford. This is vaguely referring to a 1500lb payload, and so on and so forth with the F-250's and F-350's. GM and Dodge follow suit with their 1500, 2500 and 3500 series trucks.
These numbers are just model designations. Actual payload capacities are often far off and vary with differences such as engine, two or four-wheel-drive, cab size and more. The increase in payload potential coincides with the addition to the overall strength of the vehicle. Additions include stronger frames, larger/heavier axles, a beefier suspension, etc. A quick way to differentiate between weight classes is by counting lug nuts. Typically, the more lugs the heavier-duty the truck.