Here's some observations in shopping for tires, as I am doing.
Related to the underinflated look, the cars came with 15x5.5 and 15x6 wheels, from the factory. Taller sidewalls of the 75 aspect ratio radials can make them look underinflated when they are not. Most all of the newer wheels are 7'-8", or wider, wheels. Which makes the sidewall more perpendicular to the road surface. Especially when compared to their tread widths!
On my '80 Newport, I wanted some wider wheels and found them in the 15x7 Magnum GT wheels, which were on factory closeout from Chrysler at the time. Those, in combination with the factory radial tire size, P215/75R-15, the sidewalls were perpendicular to the ground, which decreased sidewall flex and sharpened the handling to more than would be suspected. Higher inflation pressures, by a few psi, I felt, was needed to keep the tread flat on the road, which worked synergistically with the other side issues to improve handling and tire life. WIN WIN as the car had the factory HD suspension (non-rear sway bar HD suspension) to start with.
As to "underinflation", that is variable. Meaning a P235/75R-15 tire can be underinflated if there is not enough air in it "to carry the load". At the old 32psi max pressure, P235/75R-15 (replaced the old L84x15 size, and 9.15x15 before that), if any larger car had enough occupants and luggage to get close to that max tire capacity at 32psi, the slightest bump would have resulted in the chassis dragging the ground and the rear bumper dragging on any inclined driveway approach, I suspect.
So, to me, the ONLY cars which really need a P235/75R-15 tire are C-body wagons and Imperials. "NEED" is the operative word here, as to things other than cosmetics or what one might desire to see . . . personal orientations. Read the tire sidewall as to weight carrying capacity, times that by 4, Then subtract 4500-5000lbs from that figure, to get the "reserve capacity" of the tires with an empty vehicle. Then subtract 5x200lbs for 5 passengers, which then leaves the result for luggage and such. To me, that is reality. Then do the same for P225/75R-15 (which dimensionally is closer to the old J78x15 tire than not) and look at the difference between that and the larger size.
The other spec to look at is the "revs/mile", which relates to odometer and speedometer accuracy. If the car did not come with a tire the size of P235/75R-15, the readings will be less than actual distance traveled, as the indicated speed will be a bit lower than actual, too. IF that matters.
As to MODERN tires, look at the tread depth in the specs! Not just the wear ratings. Seems that some newer rubber compounds are allowing the tire companies to skimp on tread depth as others are not skimping. Look for 11/32" tread depth, which used to be the norm when the cars were built. Seems like the Hankooks are at 9.5/32" as the Nexens are much closer to 11/32"? This is important for WET TRACTION, not just longevity.
In this respect, most of the approx 18" tires are on the thinner side of tread depth, when new, too! Which means tread designs become more important, so water can be evacuated from under the tread as it rolls on the roadway.
The last issue is inflation pressure. The "standards" used to be that 24psi was for a soft ride, even when the tire capacity at that inflation level would tolerate added passengers and luggage. Reading further in the owner's manual of my '70 Dodge Monaco Brougham, it stated that "For speeds over 75mph", to add 4psi, which put things at 28psi, cold minimum. So that was what I used for the rear tires, adding another 2psi to the front to compensate for the added weight on the front of the car. Which made the steering a bit more responsive at the same time. With the newer P-Metric sizing, I ended up with about 32/30. All of the tire treads wore "flat" and lasted longer, too.
"Underinflated" look? Just part of the deal with any radial tire, typically. Especially with the older 5.5"-6.0" wide wheels. Check the air pressure and keep it near 35psi cold.
Sorry for the length. Just my experiences and observations.
CBODY67
