Tire wizards - please share your knowledge

[Guest quadfins]

Other posts in a separate forum got me to thinking about how tire nomenclature has changed over the years. Back in the first Reagan administration, when I set out to buy my own first set of new tires, I remember the choices as being between E78-14 and F78-14. By the time I went shopping again, P-metric sizes had been adopted.

Concerning the alpha-numeric sizes, is it true that the letter referred to the load rating, and the number referred to the relationship between width and height?

When one goes shopping at Tire Planet now, there are any number of width and height combinations to match any wheel size ? p19560R14, p20570R14 etc? The choices seem endless.

Now, I don?t recall there being so many different options back in the day. I only remember the different letters and rim sizes, but they all seemed to be 78s. Were there not as many choices then, or was I just not paying attention.

And another one. My factory build sheet says my car originally had 8.20 X15 tires. I presume that the 15 stood for the rim size, but what did the 8.20 refer to? And how did one determine the height to width ratio? Was there a ratio? Was there any selection, or were tire limited to a very few size offerings? Did you just get tires according to the rim size and weight of the car? How wide were the treads? Was there any choice or flexibility (besides white or black wall)?

[Mike Cullen]

I'm not an "expert" by any means. A cheap and dirty explanation is:

Letter sizes: A-L is tread width, the next number is aspect ratio 78 was standard and closely matched the old number sizes. (the number sizes were an aspect ratio of 80-85 depending on the maker) and of course the last number is rim diameter. example: G78-15, G70-15; the 70 would be a slightly lower profile tire, about the same width. adding an R designates a radial tire, GR70-15 adding a B designates a belted tire, GR70-B15

the older "number size" tires are tread width and rim diameter, aspect ratio was different by manufacturer and style.

example: 7.50-16 7 1/2" wide, 16" rim. height was generally 80-85% of tread width, no real standard.

metric: number is cross section (distance around tire bead to bead)in mm. second number is aspect ratio, next letter is speed rating, then letter R or B, for radial or belted bias, then the final number is rim size. example: P225-60ZR-17

designation means: 225mm bead to bead, sidewall is 60% as high as tread width, speed rated Z, 134 mph+, radial construction, 17" rim. aspect ratio etc. is now more critical due to electronic controls, etc. back in the day, the only problem with changing tire size was speedometer inaccuracy. Today's cars can have all sorts of driveability problems induced by changing tire size.

[Guest quadfins]

Most interesting and helpful. Thanks!!!

So the difference between the standard 8.00 X 15 and the optional 8.20 X 15 is the optional is slightly wider (besides also being a whitewall, which is not indicated in the coding). And the load rating is not indicated, so you'd just have to trust the salesman?s word that the weight of you car would not cause a blowout. Same for letter series, too.

Is there any standard formula for the letters width? For example, I can figure out that a modern 200 tire is 5 mm wider than a 195. What were the increments between letters? 1/2 inch, 1/4 inch? And where did they start as a minimum?

[Mike Cullen]

Weight capacity per tire should be on the sidewall of the tire near the bead, as is the ply rating. As far as the width increment on the letter sizes, I can't remember where I saw it, one of the tire company websites should have that information, if I recall, it wasn't a uniform progression.

[Guest quadfins]

And what are the most reputable souces for original style bias ply tires?

[ex98thdrill]

Rick Hoover is usually working for Coker Tire at Hershey. If we can get him on the forum, he might be able to explain it for you.

As for the tires, Coker may stock the tires you need.

[Peter J.Heizmann]

quadfins...

Take Pat's (ex98thdrill) suggestion.

Within the immediate left column of this AACA Forum are the "very" valuable supporters that make this site possible.

Your "quest" questions can be answered correctly to include safety, not to mention, you will support this AACA Forum.

My 2-bits (with inflation, my 2-cents is history.)

PJH... <img src="http://forums.aaca.org/images/graemlins/grin.gif" alt="" />