They do make super singles for the 19.5" rims and some are rated well above 7,000 pounds required for a 'dually' pickup. The 4X4 F-650 has super singles on them (aftermarket) and are huge, with treads about 1.5" deep! They look about 15" wide.
The super singles in the picture at the top on the blue class 8 truck are probably rated around 10,000 pounds each, to match up with the normal 34,000 pound axle rating for the two rear axles. (Combined 2 axe weight rating is normally about 34,000 pounds on a class 8 truck).
Cement trucks have been using super singles for many years.
Fred.