What is the most air pressure you can put in your tires for the best mpg and longest tread wear?

 

Never exceed the max psi listed on the tires sidewall. Sometimes running the air pressure really high can cause the center portion of the tread to wear faster. With most truck tires the 40 to 50 psi range will work best.

 

Wandell is correct. Not enough info to give a complete answer in your case, so here is what I have learned over the years. These pressures are for on an F150, and not even the F150 with the heavy payload (97-03 7700, 04 up 8200).

P series tire with a max of 35 psi - run max. The tire is at it's limits.
P series tire with a max of 44 (extra load) - run 35-38
LT load Range C, max 50 psi - run 40 (might be able to go a coup-le of pounds lower on an03 or earlier F150 as they are lighter, but only a couple)
LT load Range D, max 50 psi - run 40 -42
LT Load Range D, max 65 psi - run around 45, no lower than 40
LT Load Range E, max 65 psi - run at least 45, would not go lower
LT Load Range E, max 80 psi - run at 50, go no lower than 45

These should get you the best compromise between gas mileage, driveability, and tire wear.

To go the max on these, which would get you the best mileage, would give you poor driveability, to the point of possibly being dangerous, especially in wet weather, and would give you the least tread life.