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Tire size measurement methods


jjvw

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The most accurate way to measure actual tire size is revolutions per miles. I use this site to compare tires:


Here are 2 tires I have used on my Jeep.
View attachment 180752

View attachment 180753
That number from Tire Rack doesn't work with the Grimmjeeper.
 

Rubicon John

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Revs per mile. That’s really where it’s at assuming the manufacturers’ information is correct and that it’s available for all tires.

Assuming these are always true, it makes measuring unnecessary.
 
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Jeff Allan

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My tire thread size from yesterday got canned. I thought it was a legit question and then everyone started arguing. Geesh...

I said that and got reemed. Glad somebody agrees

Yep, in yesterday’s thread. I measured my KO2 spare today, and it’s virtually the same as the ones under load.

This is like jeans sizes. I’m in good shape, wear a 32” waist, but you put a tape measure around you and it ain’t 32”!

They all lie!! Ha ha
 
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pagrey

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Obviously the numbers they quote aren't exactly right, they list 32.5" which is 621 rev/mile and 640 rev/mile which would be 31.5" and it obviously ain't both.
 
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andy29847

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Obviously the numbers they quote aren't exactly right, they list 32.5" which is 621 rev/mile and 640 rev/mile which would be 31.5" and it obviously ain't both.
Maybe they test it loaded. Regardless, if they do them all the same, then it is a valid comparison. IIRC, the 621 revolutions per mile that equals 31.5" is what started the closed thread yesterday. :)
 
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Mr. Bills

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Once we know this true tire size, what do we do with that knowledge?
Bludgeon people with the number on Internet jeep forums?

Use it to play with online gear calculators when there's nothing good on TV?

Actually, #2 is a legitimate use, assuming one really wants to use tire diameter (calculated or measured) vs. revs per mile or the "loaded radius x 2" in that gear calculator.

Personally, I do not use the measured or published tire diameters in gear calculators - I use the published revolutions/mile if available or a measured loaded radius at street psi x2. Using published tire diameter or "industry standard" diameter for a tire particular size is a last resort that produces inaccurate results in gear calculators that while entertaining are no closer to reality than "someplace in the ballpark." This is so because those diameters fail to take into consideration the loaded radius, which is what axle gears actually "see." [Harry Lewellyn of Coyote Enterprises often observes that airing down is like going to a lower effective axle gear ratio*.]

Other than gear calculators I cannot think of a single time in the past 50+ years that I've been driving jeeps that I needed to know the actual tire diameter in inches or mm. If I want to know if a tire fits the way I want it to I have it mounted and see for myself; I try not to rely entirely upon manufacturer's claims or internet reports that those tires on Cousin Bubba's jeep fit just fine or that its a "real" 33, etc.

Wait, there was one time. I measured the approximate diameter of my spare on my old CJ to be sure a fugly spare tire cover that a friend wanted to give me would fit. (It wouldn't, thank goodness, so we were all able to save face.)



________________________________
* See, https://www.coyoteents.com/stuck-in-sand/
 
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jjvw

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Maybe they test it loaded. Regardless, if they do them all the same, then it is a valid comparison. IIRC, the 621 revolutions per mile that equals 31.5" is what started the closed thread yesterday. :)
As of last Sunday at a 1/2 tank of gas, my Jeep weighs 4,560lbs with a near 50/50 split front to rear. According to my gauge, I get the 35" tires filled up to 24psi. Is this what my tires were tested at for the marketing material?

Incidentally, my hub height (loaded radius) is 15.75". Ponder that measurement for a moment. Someone with a heavier or lighter Jeep should compare their's to mine too see what kind of consistency we see.
 
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mrblaine

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I'd considered that, but the way the OP took the time to type it out in a thoughtful way, it didn't strike me as actually being a troll thread.

I'd just like it if threads like this didn't always have to turn into a pissing match.
They will stop being a pissing match when folks stop being retarded and using terms like "true 35" tire". Until then, get out your piss proof gaiters.
 

mrblaine

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My Mickey Thompson 35's shrink to 27's at that pressure.

I hate shrinkage. ;)
We once competed in CRCA against a stretched CJ with 44" Boggers on the rear axle and 37's on the front axle. The tire size limit in our class was 35's.

There was a CRCA forum where the rules and issues could be discussed. How the 44" tire guy was allowed in our class discussion was nearly identical to all of these tire size discussions so far. Pretty much bullshit since everyone had a way of measuring the tire that had exactly zero to do with its stated size on the sidewall or its actual diameter. I ended my participation in that discussion with the parting shot " Ya'll can let the fucktard air that bitch down until it has a measured height of 35" but that won't ever stop it from being a fucking 44" tire". If you don't institute a rule change that states a tire has to be measured horizontally for size, ya'll can fuck right off."
 

rasband

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They will stop being a pissing match when folks stop being retarded and using terms like "true 35" tire". Until then, get out your piss proof gaiters.
If you’re referencing my post #3 the comment was meant tongue in cheek - people get really sensitive about tire sizes for some reason and use the phrase “true <dimension>” as if it’s the end all or even important without consideration of what they’re really looking to accomplish.

If you’re speaking more generally, agreed.
 

mrblaine

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If you’re referencing my post #3 the comment was meant tongue in cheek - people get really sensitive about tire sizes for some reason and use the phrase “true <dimension>” as if it’s the end all or even important without consideration of what they’re really looking to accomplish.

If you’re speaking more generally, agreed.
I'm sensitive to the "true" thing being used. The TJ Dana 44 front or rear isn't a true Dana 44. Yeah bullshit. There is no such thing as a true 44. Manufacturer calls up Dana, sends over a set of specs for an axle that uses a Dana 44 gear set and they build it. That makes it a Dana 44 or whatever they decide to call it. Stop with the "true" bullshit already.
 
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rasband

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I'm sensitive to the "true" thing being used. The TJ Dana 44 front or rear isn't a true Dana 44. Yeah bullshit. There is no such thing as a true 44. Manufacturer calls up Dana, sends over a set of specs for an axle that uses a Dana 44 gear set and they build it. That makes it a Dana 44 or whatever they decide to call it. Stop with the "true" bullshit already.
That was the part I held back, many folks use it in this context as an elitist term.