FIRST, there were numbers to denote the tire's section width and rim diameter. No mention of load ratings. As 8.00x14
Second, came the lower-wider tires of about 1966, as in 8.25x14 (repl the prior 8.00x14).
Third, then came the AlphaNumeric sizes in about 1969 or so. The letter indicated "Load Rating", but not with respect to the number of plies in the carcass of the tire. As in H78x14 ("H" = load rating, "78" = Aspect ratio, "14" = rim diameter).
The load rating numbers went from "A" to "L", with the aspect ratios varying from "78" down to "50" at that time. The combination of load rating and aspect ratio determined the diameter of the tire (i.e., "Revs per mile"). The largest tire Chrysler used OEM was L84x15, but others just used L78x15, but with "Extra Load" capabilities (i.e., 6-ply rated and inflation pressures to 36psi rather than 32psi).
Fourth, came the P=Metric sizing. Part of the seemingly up=coming "metric age"? 1 inch = 25.4 millimeters. Normal inflation pressures went to 35psi rather than the prior 32psi for alleged better fuel economy. As some others were rated to 45psi with no increase in load capacity, by observation. As in P225/75R-15.
"P" = P-metric sizing "225" = section width in millimeters "75" = Aspect Ratio "R" = radial, but "D" meant "bias ply/diagonal" "15" = rim diameter
The double-stripe (equal widths) white sidewalls seemed to be specific to 1970 and 1971 C-body cars. The H78x15 spare that was in my '70 Monaco Brougham was a H78x15 Firestone Belted tire. It had some wear on it, so was probably the best one of the orig tires when the new tires were purchased.
Chrysler products ususually had Goodyear tires on them. The pictured Goodyear Custom Power Cushion Belted was the OEM-level tire of that time and is what our 1972 Chrysler Newport Royal came with (a factory a/c car), but a single whitewall of about .75" wide.
Chrysler and others used the double or triple-stripe whitewalls for several years in the later 1960s to early 1970s as "something special" to do, but these special sidewall treatments gave way to the normal approx .75" wide whitewalls of the later 1970s. By that time, the nostalgia deals were operative and some Chrysler top-level cars had whitewalls that were a bit wider, like 1.0" wide for an extra-luxury look. Over time, those went away, too, leaving only the "normal" .75" whitewalls for replacement.
When the P-metric "fuel economy" tires came online, they were also lighter weight. Comparing a prior H78x15 tire to a P225/75R-15 tire, the later one is several pounds lighter than the H78x15 tire. From about 30-32lbs of tire weight down to about 27 pounds of tire weight in EACH tire. The only real place they could remove the weight was in the tire sidewall area, although the tread area is probably a bit thinner, too. It should also be mentioned that many of the newer 20" tires have about 8/32" of total tread depth, down from 11 or 12/32" for "old design" tires. But with the same mileage ratings for the newer tires! Which means a modern, low-rolling resistance rubber in them, BUT that thinner tread CAN result in some wet weather traction issues on the Interstate, alomg with some plus and minus side issues, too. Which can be where the "tire spec" charts at
www.TireRack.com can come in handy!
When all we had were "bias ply" tires, nobody worried about how good they were at high speed uses. Just that the faster you were going to run them, or how much weight the car/pickup truck had in it, the more air you put in them, up to the recommended 32psi for a 4-ply rated tire. Nobody worried about "tire age" back then as the tires would normally last about 35K miles at best. Maybe up to 45K if you took really good care of them and drove easy?
In the later 1960s, "bias-belted" tires came out as a transition between bias ply and radial tires. A hybrid of sorts. The benefits of a more stable tread without the added cost or ride harshness (at lower speeds) of the true radials (as in Michelin X). BFG and UniRoyal had USA radials which were either aftermarket or OEM, starting in about 1967-1968 on some GM cars, with Ford having optional Michelins on some 1969 models.
Keeping up with the new tire technologies and models was almost as time-consuming as keeping up with the new cars each year, back then! A really fun time to be a car person!
Some observations . . . when the P-Metric radials came out, there were some internal tweaks which took out some of the 45mph ride harshness that some cars were sensitive to. Chryslers ususally a bit more than others, due to their UniBody construction rather than "body on frame" (i.e., most Ford and GM cars). This also seemed to take out a bit of the "radial feel" of the earlier Michelin X tires, too. The earlier cut-away illustrations showed the carcass plies running at a 90 degree angle to the rim, with the "belt plies" running at a 90 degree angle to the carcass plies. More recent P-metric cut-aways show a slight angle between the belt plies with then no always being parallel to each other, but with maybe a 10 degree angle, for better impact harshness reduction, I suspect. Not unlike the belts in the bias-belted tires, by observation. Interestingly, many brands now use "a cap" on top of their normal radial belt plies, for added high-speed durability. This cap generally run parallel with the wheel rim's bead.
In modern/current times, I don't know that anything other than an OEM-brand radial would be good for a long trip, as we used to take 2-week family vaciations back then. It seems to me that repro tires might not be of the same exact weight and contruction as they were when new. Which means use a readily-available tire for daily use and leave the expensive repro tires for shows and those who can afford almost $1k/set in tires that look correct for their vehicle. Which is one reason the whitewall Hankook Kinergy tires were so good to see!
For the progression of tire sizes on Chrysler products in the later 1960s to earlier 1970s, check out the dealer order guides for standard and optional tires at
www.hamtramck-historical.com.
Enjoy!
CBODY67
