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Tire sizes explained

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Scratch View Drop Down
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    Posted: 29 June 2022 at 10:15am
I know that for example a 275/60R16 means: 275mm is the width of the tire measured from sidewall to sidewall, the 16 is the rim size, the R means Radial, and the 60 means the sidewall is 60% the size of the tread measurement.  Simple.  I have to do some math, but all the measurements are there.

I also understand that us Americans refuse to live by the metric system so a 31x10.50-15 means a 31" tall tire, it's 10.5" wide and has a 15" rim.
Even more simple. (for me at least)

But I'm not sure I understand what the Q78 means in this:
Q78/x15

Or what the 7.5 means in this:
7.5x16

I'm assuming the 78 is mm and is probably the same measurement as the 7.5 but is that the sidewall height or is it the tread width?  If it's tread width, (which I think it is) that would mean that a 7.5x16 and a 7.00x16 should (or could) be the same overall height, or have the same sidewall height... but they don't!
It's missing a measurement isn't it?

And what does the Q mean?


Are they making things difficult, or is it just me?




Edited by Scratch - 29 June 2022 at 12:03pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bight Quote  Post ReplyReply Direct Link To This Post Posted: 29 June 2022 at 11:06am
thanks for asking. more of us await answers. 

i asked this question over in oldpappys thread:

is a 7.0x15 tire the same circumference/diameter/height as a 7.0x16? 

yes, different rim/wheel size/diameter. 


Edited by bight - 29 June 2022 at 11:26am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote otto Quote  Post ReplyReply Direct Link To This Post Posted: 29 June 2022 at 11:08am
The 78 is the aspect ratio- the Q is anyone’s guess. Probably whatever the tire company wanted it to be. The further through the alphabet, the larger the tire. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Oldpappy Quote  Post ReplyReply Direct Link To This Post Posted: 29 June 2022 at 11:12am
The 7.50 X 16" I have are certainly wider than 7.00 X 16" so I assumed the 7.50 or 7.00 refers to tread width, but they are also taller. I haven't measured height from rim edge to tread but wonder if the 7.50 refers to both tread width and sidewall height? 

I'll head down to the barn and measure some more. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bruce W Quote  Post ReplyReply Direct Link To This Post Posted: 29 June 2022 at 11:41am
  It’s not tread width, it’s “section width”, a measurement thru the tire from sidewall to sidewall. “78” is the “aspect ratio”; the tire from the rim to the face of the tread is 78% of the section width. Most tires in the old days, bias-ply cotton tires with the old 6.00-16, 7.50-15, etc had an aspect ratio of 83%. When “low-profile” tires appeared in the ‘60s, the letter designations began. F70, F designated the section width, 70 was the aspect ratio. A G60-15 tire was about the same height as an F70-15, but much wider. Larger section width but smaller % aspect ratio. Normal, non-high-performance, letter-designated tires had a 78 aspect ratio. In the case of the 31-10.50 15, yes 31 is the overall diameter but the 10.50 is, again, the section width, not the tread width. 
  This is what I was taught when I was in the tire business, but it’s been a long time 😉. BW 


Edited by Bruce W - 29 June 2022 at 11:43am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldtime Quote  Post ReplyReply Direct Link To This Post Posted: 29 June 2022 at 12:00pm
Although it’s very important for jeeps the actual tread width is never supplied on tire designations.
Only the section width (widest point of the sidewall) and the aspect ratio is always noted.
Early tires designated as something like 6.00 x 16” are alway at 100% aspect ratio. That 100% aspect ratio is what makes them look so skinny.
On the 1960’s they came out with the 78 aspect ratio which at that time became the new standard.
The 78 aspect ratio was selected because test showed it to be optimum. Meaning that sidewall flex during high speed cornering was kept to a minimum yet sidewall flex to absorb impact shock remained very good.
Because jeeps are really not improved via high speed cornering the 100% aspect ratio is optimum.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Scratch Quote  Post ReplyReply Direct Link To This Post Posted: 29 June 2022 at 12:06pm
I like how on the TireRack website, they say this:


which explains that Section width is sidewall to sidewall... but then they also say this, which shows an image of measuring just the tread... Confused








Edited by Scratch - 29 June 2022 at 12:08pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bight Quote  Post ReplyReply Direct Link To This Post Posted: 29 June 2022 at 12:09pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Scratch Quote  Post ReplyReply Direct Link To This Post Posted: 29 June 2022 at 2:03pm
Very helpful, thanks!

Here are the important bits I see for us old jeep owners:

Numeric sizing is most common on bias ply tires manufactured from the late 1920’s through the late 1960’s. When this new style tire and wheel hit the market, it had a new sizing structure that we now refer to as numerical sizing. It leads off with the tire's section width, in inches and then features its rim diameter (ex. 4.50-21, sometimes referred to as 4.50x21).

As a general rule of thumb, tires from the 1930's through the 1950's feature a 90 percent aspect ratio, meaning that the sidewall section height is 90 percent of the tire's section width. Any tire size that ends with a zero, such as 6.70-15 or 7.10-15 featured this approximate high percent aspect ratio. By the mid 1960's, tire sizes transformed again. Any bias ply tire ending in the number five, such as 6.95-14, 7.75-15, etc. featured an 80 to 84 percent aspect ratio.

So we now know that the 7 in 7.00x16 is the section width, and the aspect ratio (sidewall height) is 90% of that?

So the wider the tire, the taller it will also be which makes sense.

Even the Q thing is explained:
Alphanumeric sizing became common in 1967. The size starts with a letter, which is the tire's load range. The higher the letter, the larger the tire. A78-13 is an example of a very small tire for an economy car, and L78-15 is an example of a large tire for a Cadillac. The two-digit number after the load range rating is the aspect ratio. The two-digit number at the end is wheel diameter.

Mystery solved.  
Except for that Tire Rack thing, but I'll chalk that up to a typo.



Edited by Scratch - 29 June 2022 at 2:29pm
43 GPW 93487 Blk
44 GPW 230283 OD
46 CJ2A 50100 Tan
46 CJ2A 77632 Wht
47 CJ2A 141681 Grn
48 CJ2A 156240 Red/Yel
48 CJ2A "Lefty" 181341 Red
61 CJ5 123120 Tan
03 TJ Inca Gld
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldtime Quote  Post ReplyReply Direct Link To This Post Posted: 29 June 2022 at 2:26pm
Modern 85 aspect ratio tires are much wider than old Jeep size tires of equal circumference.
Other sources clearly state the typical aspect ratio of Jeep size tires very near to 100% aspect ratio.
I’n other words the section width on a 6.00 x 16” is very near the same as the sidewall height.
That said all mfgrs will vary to some degree.


Edited by oldtime - 29 June 2022 at 2:28pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bight Quote  Post ReplyReply Direct Link To This Post Posted: 30 June 2022 at 3:22pm
measured my 6.00-16 LT NDT's (not while on vehicle)(16 x 4.5 rim)


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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldtime Quote  Post ReplyReply Direct Link To This Post Posted: 30 June 2022 at 3:40pm
Yeah that makes sense and it seems to be around 89% aspect ratio.
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