The MAX PSI cold-inflation level for your tires is stated in lettering on the sidewall of the tires.
LT235/75 R15 (Canada)
34 inches beoch
Lt235/85/r16
My 99 F-250 SD came with LT235/75R16, but it will take up to LT275/75R16 on the stock 16" wheels.
My 1969 F-250 4X4 has stock LT235/85 R16 that have plenty of room in the wheel wells.
There should be a sticker inside the driver door that tells the OE size. What will work is LT245/75/16, LT265/75/16, or if it has the 16 X 7 wheels....LT235/85/16 all sizes in no less than 10ply! Some tire sizes have their advantages. What are you using the truck for? Some tires or tread designs are only made in specific sizes.