Many decades ago, an international organization called the "Tire and Rim Manufacturers Association" published an very informative "tire and rim manual" that contained detailed information about tire and rim dimensions and load capacities. This was back when "bias ply" tires were the norm, and when "typical drivers" went about 5 or 6 thousand miles per year, and 10,000 miles per year was fairly rare.

The practice among auto manufacturers then was to size tires on new passenger vehicles to get an "average tire life" of 10,000 miles.

Since reducing "unsprung weight" by using the lightest possible tires (and wheels) helps to give a smoother ride, and since smaller tires were a bit cheaper, the tendency was to deliver new vehicles with tires that were, by one criterion, "undersized and overloaded" and that wore the tread off fairly quickly.

Predictably, a tire sized and inflated to wear uniformly across the width of the tread gives the maximum "miles per tread;" but the tire can be safely(?) "overloaded" within limits so that the only real result is that you wear off only part of the tread. When that part of the tread is gone, you need a new tire (a bit sooner).

I had access to "the book" ca. 1965 when my "new" car needed a full new set, at the predictable 10,200 miles. With a little research in the book, I found that I could step up from the original 6.70x15 tires to 7.10x15 (on the same rims), and with the larger tires I could expect "flat tread wear" and maximum tread life. My second set of tires on that car - with no other change - ran for 36,000 to 42,000 miles before replacements were needed based on the same "remaining tread depth" at end of life.

I did have to change the speedometer drive gear to compensate for the change in tire effective rolling radius, since the new tires gave a reading about 11% "off" without the adjustment. (An indicated 60 mph at an actual 67 mph could result in lots of tickets?)

This was at about the time that the media were touting a change to radial ply tires "because they last longer." My little experiment demonstrated that there was no significant difference between tread life if bias ply and radial ply tires were properly rated based on the same tread life criteria. There are handling differences between radial and bias ply tires that are of some significance; but it's difficult to argue that those differences were of much real significance to "average drivers" of that time.

The real reason - then - for switching to radial ply tires is that the manufacturing process for radial ply tires is much more easily automated, which reduces labor cost per tire.

(It should be noted that the differences in handling do require "retuning" the suspension for optimum performance with the type - bias or radial - intended. All current/recent vehicles probably are "tuned" for radials; and bias ply tires - should any such be found - should not be installed on them.)

Sometime ca. 1968 Congress decided that they knew more about tires than the engineers did, so massive new regulations were imposed. To avoid "publicly contradicting" the legislators (which might incur legal liabilities), the TRMA book is, so far as I can tell, no longer published. That makes it very much more difficult to find accurate data on which users can base modifications to their tire application or can learn about proper use and maintenance of the tires that the manufacturers specify for their vehicles.

It is fortunate that new materials and more consistent manufacturing methods now produce somewhat more consistent tire products, so fairly generic rules suffice for average users of most vehicles.

Tire failures still occur, however, because many (most?) drivers are not really as "average" as they think they are, and ignore practices that should be observed for their "unusual use" of their vehicles.

John