My go-to reference for aircraft tires is the Michelin Aircraft Tire Databook
. Most of what is found there will apply to other brands in the same sizes. I winnowed the little chart below for 5.00-5 main tires.
Note that inflation pressures may be adjusted downward in proportion to load. For example, a 5.00-5 4-ply has a 31 psi max pressure when unloaded
. Max static load is 800 lbs per tire. If we assume 90% of the load is on the mains, an RV-12 at 1320 lbs would load each tire...
(1320 x 0.9) / 2 = 594 lbs
(594 x 31) / 800 = 23 psi minimum pressure to achieve design spec, which is mostly a matter of how much the tire can compress before the rim and bead mash into the carcass. See the "bottoming load" values.
For an RV-12 at 1320 lbs, a 4 ply rating is more than enough. An 1800 lb airplane could maybe
use the 4 ply rated tire, but it would be right at the maximum allowable load...
(1800 x 0.9) / 2 = 810 lbs
and thus would need a slight boost in tire pressure:
(810 x 31) / 800 = 31.3 psi
A 6 ply tire on an 1800 lb airplane can run an unloaded tire pressure as low as....
(810 x 50) / 1285 = 31.5 psi minimum pressure to achieve design spec
Note that minimum pressures for both 4 and 6 ply are (as a practical matter) the same, given the same load. That's not a coincidence; they're dimensionally the same. The difference is burst pressure (maximum strength, if you will), not operating pressure.
Remember two details. First, these are unloaded pressures. Add 4% for rated load.
Second, these are tire manufacturer's numbers. An airframe designer or wheel engineer may specify something different for reasons of their own, and as an EAB operator, you also have a say. For example, as Scott noted recently, a higher than specified tire pressure may increase airframe component load, given a maximum smash into the runway. A landing gear analysis says running tire pressure at max (for example, a 6-ply at 50 psi) does in fact increase component load, but the actual difference is not
large. Me? I'll take the higher pressure and try not to crash into the runway with maximum gusto.