I've had a little experience with this in the industry and will try to explain why some numbers may not always seem to make sense.
1) 2x individual tire carrying capacity at specified pressure must always equal or exceed Placard Front and Rear GAWR's
2) FGAWR + RGAWR must always equal or exceed GVWR
3) Over the years FMVSS (Federal Motor Vehicle Safety Standards) rules governing calculated vehicle curb weight (~unloaded weight), and their interpretation, have changed in terms of allowable driver and fuel weight; so, comparing vehicles across model years may be affected by this
4) Payload doesn't always equal GVWR-curb weight (whatever that may be) because payload is typically biased over the rear axle. The governing factor normally is at what "reasonably distributed" payload the RGAWR is tripped.
Add to this the fact that the manufacturers compete with one another on some of these numbers, pressuring the vehicle engineers to press the envelope on what is a "reasonable" distribution of loading of the payload. This becomes apparent when trailer towing and tongue weights come into play. Even with good equalizing hitches, tongue weight puts more load on the rear axle than it's dead weight due to "lifting" of the vehicle forward of the rear axle.
Bottom line, I will repeat what has been written in this forum many times, when in doubt, weigh your vehicle by axle and don't exceed the GAWR's. Don't feel bad if you can't load your Expedition full of people and luggage, and safely tow a large 2 axle travel trailer.....just step up to a Super Duty.
