Furthermore;
The real-life section-width will only match the spec when the tire is mounted on a rim that is 70% of the branded size.
Furthemore, all numbers are rounded to the nearest 5mm.
That means;
that a 275/60-15 might have a measured section of between 271 and 279 on the 70% rim which is .7 x275=192.5, which is 7.56, rounds down to a 7.5" rim.
OK so 271 is 10.67 and 279 is 10.98; giving a section range of 10.67 to 10.98 on a 7.5" rim..........
The sidewall height will then be; 60% of that, so 6.40 to 6.59. Multiplied by two and adding 15, I get a height of; 27.8 to 28.18.............
What's it mean?
Well, the manufacturer mounted his tire on a 7.5" rim, and measured it. If the section fell between 271 and 279; it got branded 275. And if the sidewall height fell between 6.50 and 6.59, then it got branded 60%.
You would think.
No that's not exactly right, because the aspect-ratio of 60% is not an exact number, but rather, just rounded to the nearest 5 like 50,55,60,65 etc.
So that 60% number on the tire could be anywhere in the range of 56% to 64%
Sheesh.............
Ok but no streeter in their right mind installs a 275/60 on a 7.5rim. That is just the spec rim.
More than likely, you will install it on an 85% rim, which is 9.20. And there ain't no 9.20 rim that I have ever seen, so that will round up to a 10", or down to likely an 8.5". I ran them on 8.4s no problem.
Ok but on an 8.5 rim, that baby is no longer a 275. The sidewalls will move larger, perhaps, I'm guessing one-half the difference from 7.5 to 8.5 which is .5 x1.0= 1/2 inch, which is ~13mm.... so the tire becomes 271 to 279 plus 13= 284 to 292.........
Ok now, even tho the tire is the exact same tire, the aspect RATIO is a RATIO, so now, that number has to reflect the new section width, so:
The height of the sidewall does not physically change , so is still 6.40 to 6.59 which is 162 to 167 , making the new aspect ratio; 162 to 167/284 to 292, lol. And the range there is now;AVERAGED to 164.5/288=57% .......
So now, we started as branded 275/60-15, on a 7.5,
which on an 8.5 might actually measure as a 292/57-15 AVERAGE. The mounted tire might measure +/- 4.999 mm in any direction!
Ain't it grand.................
But it gets worse;
Because the height is ethereal, and it gets multiplied by 3.1416 to get your tire roll-out, your tire roll-out will affect your speed-O by the variance or +/- 15.67mm..... On the 292/57-15, the roll out is, 88.296 inches +/- .617 inches...... which is 87.68 to 88.91 or 1.23 inches from min to max. or 1.41% or, ~1mph @65 mph.
EDIT
OOps I noticed that some of my math is off a tad, not enough to be meaningful, so I'm not gonna redo it; The point is all the same; Until you install a tire on your chosen rim, and MEASURE it; all bets are off.