[Blue Bowtie]

For a specific TPC on a specific vehicle, there is a pretty darned good chance that the engineering performed by the factory to determine correct tire pressures for a given position (front/rear) are about as close as it will get to being right.

Start changing the TPC (tire design), wheel width, and possibly the vehicle loading, and all of that becomes merely a guideline to correct pressures.

As an example, I have a 2003 S10 4X4 with Z85 suspension. Tire options were the "standard" Goodyear LT/M&S 4/2 ply radials rated for 1,609 lbs. at 35 PSI (TPC 105S), or optional Goodyear AT/M&S 6/4 ply radials rated for 1,984 lbs. (TPC 104Q). That latter set was also capable of off-roading at 18 PSI inflation or "normal" use at 50 PSI.

If another brand or TPS (or load index) tire is mounted to that, all bets are off, regardless of the 235/75-R15 size match to the originals.

I think you are probably correct that the factory had their numbers right for those original designations.