One correction in my Post #20, the radials I initially put on my '67 Newport were Kelly-Springfield fiberglass-belted radials in P225/75R 14 size rather than P225/75R-15, as stated.
In digging around in the Hamtramk-Historical website's Library section, I found the old spec sheets which Chrysler supplied to the Automotible Manufacturers' Association for many models and model years of cars. In the tire section, I was able to find the "Reva/mile" specs for many then-OEM tire sizes, std and optional.
In particular, the H78-15 tire size revs/mile was 728. Additionally, G78-14 = 768, G78-15 = 746, GR78-15 = 744, HR78-15 = 727, J78-15 = 715, LR78-15 = 704, L84-15 = 695. The "revs/mile" spec is important for correct speed/distance calibration of the vehicle's speedometer. A tire article in Car Life mentioned that each of the alpha-numeric sizes (at that time, usually "78" and "75", to later include "70" and "60", but basically the normal mainstream sizes) had a government size spec, which would relate to revs/mile, from which there could be a 7% variation.
The alpha-numeric tire sizes were related to weight-carrying capacity (increasing with the progression through the alphabet, A-N), the aspect ratio (78, 70, and 60, initially), and rim diameter. This was oriented toward consumer "ease of comparison" of one tire size to another. Plus not putting less weight-carrying-capacity tires on a vehicle than should be there, which is where the Tire Size/Inflation Pressure decal in the lh door frame came into play, too. Of course, the lower the aspect ratio, the general idea that the tire was "higher performance" than the '75" aspect ratio tires, yet still had similar max weight capacity ratings.
The alphabetic notations for "Load Range Ratings" were another allegedly easier way to assist the consumer. Load Range "B" would be the normal "4-ply rating", "C" would be 6-ply, "D" would be "8-ply", and "E" would be "10 ply". "C" seemed to be later replaced by "XL" after the size designation, with inflation pressures of 36psi for alpha-numeric sizes and higher than that for the later P-Metric "XL" designation radials.
So you can use the above AMA Spec revs/mile numbers to shop for tires in the 16" or 17" rim diameters to replace current 15" tire sizes, or to verify which 15" tire sizes to consider.
Enjoy!
CBODY67