According to a recent Automotive News article by Larry Vellequette , the reason the Big Three pickup trucks all measure towing capacity in different ways is because Ford went back on a pledge to move to a standard promulgated by the Society of Automotive Engineers (SAE).
Mr. Vellequette wrote that automakers adopted the SAE J2807 towing test standard in 2009, and agreed to implement it in 2013 in their product literature.
Toyota started out by measuring the Tundra under J2807 when it was launched in 2010 (as a 2011 model); its tow ratings dropped by up to 400 pounds. GM had the new tow ratings in its product literature and vehicle markings as it got ready to launch the 2013 Silverado and Sierra pickups, as GM, Ford, and Ram had agreed; but just before the 2013 launch, Ford announced it would not change its ratings until the 2015 model year.
GM quickly retracted its marketing materials and reprinted owners' manuals. It is launching new pickups for the 2014 model year, but had, as agreed, recalibrated its ratings for the 2013s.
Ram used the test procedures and said any adjustment on the 2013 Ram would be small if they were adopted, but said Ram was following GM and Ford's lead.
The J2807 standard includes numerous tests, described in detail in the article. It applies to any vehicle with a gross capacity of 13,000 pounds or less; originally, J2807 had a 16,000 pound cutoff, but it was reduced to exclude heavy duty pickups.
Mr. Vellequette pointed out that similar horseplay had taken place in the 1960s, with regard to horsepower ratings, and with gas mileage claims, in the 2000s. In both cases, the government stepped in.