.
Tow ratings and how they are determined:
http://www.trucktrend.com/news/163-0910 ... -capacity/
Informative paragraph:
"Capability Testing
When towing a trailer of any size, you need enough engine torque to get it launched from a stop, including uphill, and enough power to keep it moving at reasonable speeds up long, steep grades and to safely pass on two-lane roads. You need sufficient engine and driveline cooling to avoid overheating coolant or lubricants up long hills and in hot weather and sufficient brake capacity and cooling to safely descend long downhill grades. You don't want the trailer fishtailing or destabilizing your tow vehicle under any conditions, and you don't want to overload your tires, wheels, suspension, frame, or body structure.
Not surprisingly, tow vehicle develop-ment and testing is extensive. It begins at the component level, in labs and on benches, continues through the subsystem level (chassis, brakes, powertrain) and culminates with complete vehicles from early prototypes through production. "We do a lot of hot-room and cold-room testing," says Chrysler's Cairns. "We have goal temperatures for critical components, so we take thermal data from fully instrumented vehicles in test cells and match that against CFD [computational fluid dynamics].
"We also do a lot of work in the wind tunnel to assure that we're getting good cooling airflow while minimizing drag. Cooling drag is big drag, so you want to make sure you have enough but not too much. [For that reason] for the first time, we have separated our light-duty from the heavy-duty truck in front-end styling, because we were able to optimize the light-duty for aerodynamics and the heavy-duty for capability."
Vehicle synthesis engineer Jason Briggs adds that Chrysler (like everyone) uses a variety of public-road test sites: "Davis Dam in Arizona, which is used in the coming SAE standards, Eisenhower Grade in Colorado, Baker Grade in California. We do climate-control testing in Death Valley and in the Southeast for heavy heat and humidity. We also do city traffic, with and without trailers, for cooling performance.""
That article is from 2009.
Since then, even more standardized testing is required.
"New SAE Standards
The upcoming SAE J2807 recommended practices will include five areas of testing: structure, propulsion, thermal, handling, and braking. The structure tests validate body, bumper, and frame strength and the hitch itself. The propulsion tests include: launching five times in five minutes, in forward and reverse, on a (very steep) 12-percent grade; zero to 30, zero-to-60, and 40-60-mph level-road acceleration tests; and minimum speed on the Davis Dam grade.
The thermal test, which is part of the long, grueling Davis Dam gradeability exercise, requires no loss of fluids and no warning alerts that would require customer action--and it must be done at 100 degrees F or hotter. The handling tests require sufficient trailer-sway damping and stability enhancing tow vehicle understeer with weight-carrying and weight-distributing hitch setups. The brake tests specify a maximum stopping distance from 20 mph (from federal regulations) and both uphill and downhill park-brake performance on a 12-percent grade."
Towing definitions 101:
http://www.curtmfg.com/page/towing101_ch4
.