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My layman’s understanding is that the limiting factors for towing capacity are usually the brakes, engine work capacity, and the transmissions ability to handle the prolonged torque without overheating. Are there other factors I’m not considering? Perhaps vehicle weight?

I’ve noticed that rated towing capacities on cars can vary significantly from one year to the next or between models. I’m assuming the reason is that manufacturers are avoiding liability.

For example the 2006 Subaru Legacy has a 2700 lbs towing capacity, while the 2006 Subaru Impreza with the same engine and transmission is 2000 lbs, and the 2006 WRX which is basically an Impreza with a more powerful engine and brakes has a 0 lbs rated capacity.

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  • $\begingroup$ Tires? also chassis strength if a tow bar can be fitted. $\endgroup$ – Solar Mike Jul 24 '20 at 20:31
  • $\begingroup$ Perhaps it just simply wasn’t tested on the WRX? $\endgroup$ – Jonathan R Swift Jul 24 '20 at 21:54
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    $\begingroup$ In the U.S., the methods are established by the SAE Tow Vehicle Trailer Rating Committee. Their recommendations are normally adopted by the Federal Register. See SAE J2807 for example. $\endgroup$ – Phil Sweet Jul 25 '20 at 3:51
  • $\begingroup$ A nonpaywall summary of the test requirements is here - fifthwheelst.com/SAE-J2807-Tow-Tests.html $\endgroup$ – Phil Sweet Jul 25 '20 at 4:02
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You have all the important factors in your list. The only thing I would add is:

-The maximum downward load exerted on the trailer hitch tongue by the trailer; this adds to the load on the rear wheels of the towing car while subtracting from the load on the front wheels- and can have seriously bad effects on handling & stability at speed.

By the way, the transmission temperature limit is set by the fluid temperature in the torque converter. In operation, the torque converter's slippage generates heating of the transmission fluid and cars designed for towing will have a transmission oil cooler installed to get rid of that heat. They will also have heavy-duty rear springs to handle the tongue load and high-capacity radiators to keep the engine from overheating.

In the old days of classic American cars, you could order a towing package that included the hitch, the transmission cooler, the high-capacity radiator, heavy springs and stronger tires from the factory.

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