That 20,000 lb maximum is for any individual axle. I am a commercial driver for 4 decades. The limit gets reduced to 17,000 per axle when 2 or more axles are in a close group. A single axle can go to 20,000 lb, but it has to be rated for it. Thus the manufacturers rating applies. Also, the type of tires on the axle applies. One can have a 20,000 lb rated axle, but if they are running tires only rated to 17,000 lb, then the tires set the maximum axle weight.
One can always buy permits that allow for going over the maximum axle limits, but they have to be made to support whatever you are permitting for. FMCSA has little to do with any of this. The limits are established by the NTSA thru Title 23 USC 127 and ISTEA froze those limits from further changes.
Not really complicated.
