In the '66-'70 owners manuals for our '66 Newport and my '70 Monaco, the base tire pressure is listed on the door decal (along with the mininum tire size for the car). Reading the owners manual, I discovered that to that base pressure, you ADD 4 psi for driving over 75mph, which puts things at an adjusted base pressure of 28psi. From there, there is the "Max Load" pressures which went up to the Max Rated Inflation Pressure of 32psi. In the '70 manual, no mention was made of whether for bias ply, belted-bias ply, or radials as to inflation pressures.
When P-metrics came out in the earlier 1980s, they had a stated max pressure of 35psi and the carcass plies for the radials were a bit softer in configuration to allow for the higher inflation pressures, from what I could tell. The higher inflation pressures were for less "flex" and higher potential fuel economy levels.
Usually, the front tires were spec'd at 2psi less than the rear tires. In '66, the front tires on the station wagon models were spec'd at 22psi, as the rears were closer to 32psi!
In the earlier 1970s, a Firestone tire salesman told me that ALL of their tires were designed to run at 26psi, no matter what. High speeds, full loads, light loads, etc. He was firm in that recommendation, back then.
Usually, front tires wore sooner than the rear tires did. As the front ends were heavier, I formulated a theory that tire pressures should relate to the load the axle was carrying. After finding a chart which had tire pressure and load capabilities in it, I discovered that starting with the "75mph pressure" (28psi) and adding 2psi would balance the loads the tires saw between the front and rear tires. Which resulted in 30 frt/28 rr. Which also resulted in even tire wear between the front and rear tires. Steering response was also better with very little ride quality decrease. This was on 14x5.5 factory wheels on the Newport and 15x6 on the Monaco.
Back when everybody was seeking to navigate the differences between P-metrics and earlier radial tires, there seemed to be some disagreements in what fit where. The "industry sources" claimed that you worried about tire load carrying ratings, which generally resulted in smaller (physically) tires getting installed and the inflation pressures used being the max inflation pressures on the sidewalls. My orientation was to go by the tire diameter, to maintain cosmetics of the earlier tires, which also kept the odometers reading more accurately, too! With the latter being where things ended up years later, by observation.
Later, I discovered the "Revs/Mile" spec in some BFG Radial T/A brochures, later in the Spec charts at TireRack.com. Which can be related to the same specs in the SAE Specs guides you can find at
www.Hamtramck-Historical.com, too.
IF your car has the 8.25x14 tire size spec, which later became G78-14, you can convert the "8.25" numbrer into metric 209.55mm, which would be a very close match to P215/75R-14. Which would result in having a tire with adequate load-carrying capabilities for your car and its equipment. That's the first step. With the second step being to aim for about 30-32psi inflation pressure in the rear tires, then add 2 more for the front tires. Which should cover a majority of the vehicle loading situations, other than trailer towing or a trunk full of heavy stuff with people in the back seat (where you might need to go to the max inflation pressure on the sidewall instead).
ALSO be aware that "air" shrinks with colder ambient temps and expands in hot ambient temps! Which means that you probably need to adjust tire pressures in the fall/winter and then again in the spring/summer months.
From my experiences . . .
Enjoy!
CBODY67