Also, just for the curious / non-believers.

 

i run 50psi in my LT and still average 18.5mpg

 

well time to get the heads butting on here.....
air pressure in and LT can be exactly the same as pressure in a P-metric tire, it will not be under inflated. The only reason to raise the pressure in an LT above the reccomended pressure for your stock tires (providing they are the same size) is to compensate for a heavy payload. for example (random sizes and psi):
p235/75R15 tire max cold psi - 35
LT235/75R15 LRD (8 ply) max cold psi - 60

both these tires can be run on a truck that calls for say 33 psi at that given pressure and not be under inflated.

the volume of space inside the tires are exactly the same, so the say 33psi will exert the same pressure on the tire (lbs of pressure per square inch) the extra plies in the tread will not effect that at all, but the extra will allow the tire to beable to support more weight at a higher pressure if needed.

your LT rating does not ad anything to sidewall strength, most tires are only 2 ply sidewalls reguardless of P or LT. there are a few excptions for example like BFG and Firestone as well as a few other manufactures now that will add either an extra layer of nylon (FS tires) or an extra layer of steel or kevlar (BGF TirGuard).

 

Not necessarily. Depends on the tires, vehicle, etc. True of all tires 30+ years ago, but not always true today. One of my other vehicles, using the factory recommend LRR tire part number, of which no other tire can compare for longevity and fuel economy, has the best contact patch around 50psi, and many drivers run them at 80psi to eek out a few more mpgs on the highway. I prefer to limit to sidewall max, so 45psi for me in that vehicle, on a tire the manufacturer recommends for 35psi (for ride comfort).

I've noticed that the truck community doesn't do much in terms of scientific testing or comparable data sharing so I have no clue what would be best. Attempting to find a low-rolling-resistance tire would likely have far greater impact than anything one can do to any random set of tires they buy from the tire store.

My weekend car with street tires also has best traction (launch wheelspin and braking distance, controlled best I can by EEC and transmission tune) at 45psi, not the 35psi recommended by Ford, from my own A-B-A testing (more like A-B-C-D-A-B-A-C-A-D-A with different values) with warmed-up tires on a 70 degree day. It seems like many vehicles today will see improved braking distances with higher pressure than in the door jamb, but the ride quality quickly suffers. I won't say this is always the case though - I don't have enough sample size to prove otherwise.

As for best fuel economy, more psi the better, but trucks with their high weight and poor aerodynamics will never see much benefit. A small hybrid with a 0.25 cd and 20ft^2 frontal area can see a 10% gain going from 35psi to 45psi. A sedan with a 0.31cd and 21.4ft^2 frontal area can see a 3-5% gain. A truck (of which I am unable to find cd and frontal area numbers, but likely 0.36-0.4cf and 30-35ft^2 of frontal area) should see under a 1% gain going from 35psi to 45psi. At 20mpg, that is 0.1-0.2mpg.

Get smaller/lighting tires and a lower stance to improve mpg. Think about why a Ford Ranger gets better fuel economy, and you know exactly which direction you must go to improve fuel economy in your truck. Lighter, smaller, lower to the ground, smaller engine, manual transmission. A standard v6 f150 2wd sits lower on smaller tires than a lifted 4x4 v8/eb truck and is capable of passing 25mpg on the highway without too much work. At higher speeds the brick aerodynamics will quickly kill fuel economy.

3-4 cylinder hybrid or small sedans for fuel savings (or motorcycle), trucks for hauling. This is why I have a 85mpg commuter, 25mpg weekend warrior, and 18mpg truck. Fuel savings, fast, haul 3+ people and cargo.