Back in the later '60s, when I was in high school and had my faithful K&E Slide Rule, CAR LIFE magazine had some great tech articles. Several on tires. Somewhere along the line, there was one which had tire capacity at incremental inflation pressures, 24psi to 32psi, which was the normal pressure range back then.
According to most road tests, also in CAR LIFE, the normal f/r weight distribution on a C-body sedan was 55/45.
On the '66 Newport, the base tire pressure was 24psi, cold. At that inflation pressure, the car's weight should not overpower the tires' capacity. For normal 'round town driving, 24psi worked fine for many.
Further reading the owner's manual, if highway speeds would be encountered, then "+4 psi" to the base 24psi, for a total of 28psi. That resulted in a "loaded" tire pressure recommendation of 28psi frt/30psi rr.
In the article on suspension tuning, tire "slip angle" was mentioned. With lower pressure in the front, the car would understeer or slip sooner than the rear (which had more pressure).
So, with some figurin', if the car was to have more neutral handling characteristics, then the front and rear tires should have similar slip angles, which would relate to their pressure bias, frt to rear.
Being that Chrysler products were the better handling cars of their time, I did some experimenting on our then-newer '66 Newport 6-Window sedan. What I sought to do was to have each set of tires support their proportional amount of weight, using air pressure differences to do that.
With the tire pressure chart and the weight distribution information, I came to the conclusion that the front tires needed a bit more air to "support the weight" than the rear tires did. My "even" situation resulted in my 30frt/28rr tire pressure bias. If there was to be more stuff in the trunk, then raise all pressures by 2psi to 32/30.
One other way I prototyped the tire pressure deal was to turn the front wheels side to side while parked, then roll the car back and look at the resulting rubber on the concrete. With 30psi in H78-14 tires on a 5.5" width rim, there was no light spot in the middle of the tire's print. With 28psi, just a bit, but 30psi made one black print on the concrete.
Popular Science magazine, circa 1968 I believe, had an article on tire pressure and how the car handled. They ran a test with the tires at 24psi, cold. On the highway, they did a lane change maneuver. They couinted the number of weight transfers the car did as a 2 lane passing maneuver was executed. Something like EIGHT weight transfers! Of course, most of these were due to the softness of the tires at the lower inflation pressure.
When they re-ran the test with 30 or 32psi, the car was more solid as the tires were stiffer with more air in them. Less steering input and less weight transfer activity.
So, I started with the highway speed recommendation 28psi and increased the front pressure to get the front's heavier weight supported equally well as the rear, which was 30psi. Having the inflation pressure customized to how much weight the tires were supporting also resulted in the same tire wear on both ends of the car! Another benefit of my tire pressure bias situation!
If the tires were over-inflated for the weight they were carrying, then there would be more wear on the center treads and less on the outer tread ribs. With "under-inflation" for the weight being carried, then the outer tread ribs were where the tread wear was, as the center section of the tread would "buckle upward". There were many illustrations of this in car maintenance documents back then . . . might check the Chrysler MasterTech courses for those things.
So, with the OEM bias ply tires, the inflation pressures I ran resulted in great tire wear and better steering response, with a firmer ride. It all worked well.
Why the desire for the lower front tire pressures by the factory? A driver will usually slow down when the front tires squal in a turn, as they slip before the rear tires, which is safer. If the rear slips first, that's oversteer and can become dangerous for the normal driver.
And then, on the '66 C-body wagons, they went with a 22psi/32psi f/r pressure recommendation (for full rated load). More pressure in the back for heavier loads, less in the front so you wouldn't try to "road race", I suspect.
The original radial tires ran the same pressure as the bias ply or later belted bias ply tires did. The modern P-metric radials came around in the earlier '80s, modified to not ride "hard" with the higher 35psi inflation pressure desired for better fuel economy.
When the Chrysler LH cars appeared, they had P225/60R-16 as the standard tire size. No options, except for the later Performance Handling Package. These tires were mounted on 7" wide rims. DEFINITLY "performance territory" compared ot the 5.5" wide rims of prior times! Recommended tire pressure was 30/30. At that pressure, everything worked, although sharper handling could be had if the pressure was 34/30. Most fwd cars' weight distribution was 60/40, so 4psi more in the front than the rear. Those P225/60R-16 on 7" wide rims came to be the standard tire/wheel combination on almost every normal car back then, including most GM and Fords.
With radials and belted bias ply tires, the outer belts keep the tread more "flat" on the road, which means they don't need air pressure to do that (as the bias ply tires do). BUT sidewall stiffness due to increased inflation pressure (in addition to any internal bead area stiffeners in the tire itself) result in crisper and more responsive steering inputs.
When I got my '80 Newport, it had the stock 15x5.5" wheels and P215/75R-15 radials on it. I scored several sets of the Magnum GT 15x7 wheels/rings/center caps/bolts when Chrysler had then on "closeout". I put one set on the Newport with some cheaper P215/75R-15 whitewalls from Sam's Club. What happened there was that with the wider rims with tires designed for 6" wide rims, the sidewalls were more vertical, with decreased "radial bulge", at any inflation pressure. With the 32/30 pressure bias, that car acted like it was on rails when cornering. A joy to drive with the factory "load carrying" HD suspension (no rear sway bar). And THAT's a trick that can work on any car, even with non-performance tires. Kinda glad I didn't put the wider and heavier 70 series tires on it as the lighter tires ride better and handle just as well.
Radials were around by about '67 or '68. Chrysler didn't offer any as optional equipment until about '70. My '70 Monaco owner's manual mentions using them and such. '74 was probably when Chrysler really started using them, with "rear sway bar" suspensions, too. Ford would put Michelin X tires on almost any car they sold. GM used UniRoyal Tiger Paw radials. Main concerns were with ride harshness, which would peak at about 45mph, then "get quiet" at highway speeds for that "radial feel" ride.
My experiences. Proceed at your own risk. Key thing is to read ALL of the OEM tire pressure recommendations rather than just one part of them.
CBODY67
