I use the chalk method to see where the best footprint is and base my
air on that and the sidewall flex in turns.  To get the best 'footprint'
on my 33x9.5 BFG muds with my light 'glass CJ7, I need 26 psi.  I find
this too soft a feel on corners due to the tall sidewall, so run closer
to 29-30 psi.

The tire shop installed them at 36 psi.  I only had 4" of center tread
touching the ground at that pressure!

The chalk method just means put a bunch of chalk marks across the tread
and drive straight for a short ways.  Then check to see how much chalk
is left.  You would be looking for 1/2" to 1" left on the outer edges
for a 12.5" wide tire for a good footprint.  You might have to
compromise up in psi like I did for handling.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
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If they are on stock rims they are way to narrow and you will never get a
good foot print at normal pressure, they should be on at least 10" wheels.