Originally Posted by Airdale
When changing Original Equipment (P) tires from P to LT you should work closely with a reputable tire dealer experienced in making such changes.
The "P-Metric" tires shipped on 1/2 ton trucks are usually an "Extra Load" (XL) and this is indicated on the sidewall. The difference between an SL tire and XL tire is the max inflation pressure. The tire is sized to easily carry the rated load of the axle/suspension on a properly loaded 1/2 ton pickup truck. Going to LT tires will usually yield no benefit and may even be a poorer performing tire in some cases.
You can't turn a 1/2 ton into a 3/4 ton by going to LT tires. Using my truck as an example, the rear axle is rated for 3800#. Both rear tires (OE P-Metric XL type and size), inflated to sidewall max pressure are capable of carrying just over 5500# or 30+% "safety margin".
When towing it helps to increase the pressure in the XL tires but no more than maximum rated cold inflation pressure. On my truck I have XL tires that are inflated to 42 lbs while towing. Handles like a dream with no sway or wander. Tires wear nice and evenly across the tread and my rear end (mine, not the trucks) is nice and comfortable.
If you don't need LT tires to carry the weight nothing is really gained by changing. Remember, it's still a 1/2 ton truck.