Before anyone gets all excited, remember that the base tire on a '66 Newport was 8.25x14, which later became G78-14, and equated to the OLD metric radial size of 205-14. IF the Newport had factory a/c, then it got 8.55x14s, which became H78-14, and 215-14 OLD metric radial sizes.
Conversion of the earlier section widths to metric numbers results in a 8.25 x 25.4mm = 209.55mm, 8.55 x 25.4mm = 217.17mm, with the station wagon 9.00x14 being 228.6mm wide.
So, from that perspective, with the P-metrics having section width numbers about "10mm" more than the OLD metric sizes, then P215/75x14s are in the hunt more than many might suspect. Plus they will certainly support the weight of a C-body car with no issues. When I could get them, I put a set of Kelly-Springfield P225/75R-14s on the '67 Newport (which were glass-belted radials, rather than steel-belted), back in the earlier '80s. They looked just fine on the stock 14x5.5" wheels.
As for weight-carrying capacity, even the old 8.55x14 size had about 7000lbs of weight capacity (4 tires), which considering the "title weight" of about 4100lbs for our '66 Newport Town Sedan, had a good safety margin at 30psi cold inflation pressure, much less the stated max inflation pressure of 32psi. The P-metrics might need to get to 35psi for similar ratings, though.
I replaced the K-S tires with some BFG Advantage T/As in P245/70R-14 size. They matched the old H78-14 size exactly, as I recall. Factoring that down to a 75 aspect ratio, probably a tad bit taller?
By observation, we've become more sensitized to seeing cars with larger wheels on them, especially with the switch to 15" wheels in '69. Compound that with the more recent (what I term) "Hot Wheels" look which some now desire . . . and the old wheels just look tiny, but that's just the way they were back when the cars were new. In a day when a 6.0" wide rim was WIDE, compared to the normal 5.0" or 5.5" rims that were on the narrower tires back then . . . and prior.
I know that when the P-metrics came online, there was some confusion about equivalent sizes for the older tires. Some said to use inflation pressures of "+3psi" for similar width numbers, but even back then, I felt the main criteria should be weight capacity rather than "numbers" on the sidewall. As things have evolved, my orientation has been generally where things ended up.
P235/70R-15 has a revs/mile spec very similar to P225/75R-15, so there are some similarities in those two sizes' fitments/interchangeabilities. The H78-15 (8.45/8.55x15) was replaced by the P225/75-15 size, according to my calculations AND they fit the cars just fine, by observation. With plenty of weight capacity for a loaded C-body. Or a '90s Buick Roadmaster with the trailer package.
Cooper did build a Trendsetter SE in the P225/75R-14 size. I discovered those on a '65 Newport at Mopar Nats one year, (1990s). They were an exact size/cosmetic match for the old Goodyear Super Power Cushion 8.55x14 whitewalls that came on our '66 Newport Town Sedan (factory a/c). When I got back home, I looked them up on their website. 4.5" tread width and all. But like other 8.55-14/H78x14 tires used on so many '60s and mid-'70s cars, that size seemed to get into a "dead zone" of tire sizes. Seems that the Korean brands are where the P215/75R-14 tir3es are now found in white wall sizes.
The images below are some I've harvested via Google. I highly suspect that both cars will have the old 8.55-14 tires on them. "Tiny" and narrow-treaded, certainly, but enough "guts" to carry the weight.
To be sure, an acceptable "upgrade" is the W23 wheels with P225/75R-15 tires on them. They look good and fit right, to me. Plenty of weight capacity, too. The P235/75R-15s are a bit "large" in diameter to me, nor is the extra weight capacity needed, to me.
When looking at revs/mile specs on the TireRack website, the P225/70R-15 size usually has a revs/mile spec of about 750 revs/mile. That's for a G70-15 equivalent tire.
This is the way it's all evolved, to me. Putting the OEM-spec size in modern radials looks small next to a current-spec 17" wheel, but they came with the now-smaller-looking tires on them from the factory. In a time when 15" wheels were only on things like Lincolns and Imperials, where the additional weight carrying capacity was needed. Or pickup trucks.
CBODY67
