Since the towing capacties I originally posted came from the Edmunds comparison of the manufacturers' published specifications for four differently equipped Chevy Silverados, and the Chevrolet website posts the identical figure; this does not seem to me like it could be an error. But, after reading Morin's response, I then pulled up an Edmunds comparison of the Ford F150, Dodge Ram 1500, Nissan Titan, and Toyota Tundra; all with V-8 motors. (http://www.edmunds.com/apps/nvc/edmunds
And this comparison also showed towing capacities of 7400 pounds for the Nissan Titan, and over 10,000 pounds for all the three other vehicles. Since these trucks are all V-8s, I then added a 2010 Toyota Tacoma 2.7 liter 4 cylinder, with automatic transmission, just for the most extreme possible comparison. And the Tacoma 2.7 liter 4 cyl came up with a 3500 pound towing capacity. So I changed the Tacoma to a 4.0 liter V-6 with automatic; and it came up with a 6500 pound towing capacity.
I understand that these published towing capacities sound unreasonably high; but since they come from 5 different manufacturers, they are definitely not errors. What I believe they are is a new way of rating vehicle towing capacity (in order to both sound more impressive, and also to include the real world variables of vehicle weight, and load carried in the truck itself). To test this theory, I subtracted the truck weight and half of the rated payload from the towing capacity. And the Ford F150 V8 came up at 5037 pounds; the Dodge Ram 1500 Hemi came up at 4010 pounds; the Nissan Titan V8 came up at 1161 pounds; the Toyota Tundra V8 came up at 4020 pounds; and the Toyota Tacoma 4.0 V6 came up at 1790 pounds.
This reminds me of the Great Horsepower Race of the 1960s; in which virtually every manufacturer published dishonest power ratings for their muscle cars.