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Tire size measurement methods


samobius

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Can we have a discussion on the best method to measure tires?

Here are my viewpoints on the matter. Measuring a tire from ground to top of the tire has issues for a few reasons:
  1. It's impossible to account for the load from one vehicle to another as a comparison for what a tire will look like on one vehicle vs another. More weight in/on a vehicle will make the tire appear shorter.
  2. Air pressure will come into play here. Higher air pressure levels will show a taller tire.
  3. Tire load range may be a factor. E load tires may not be as affected by the above

So that leaves us with two questions
  1. What is the best way to measure a tire that eliminates those factors?
  2. What is the most accurate way to measure that ensures you're not affected by a parallax issue?

In my opinion, the best answer to those questions is by measuring the circumference of an unweighted tire and then using d=C/π solve for the diameter, where d=diameter, C=circumference and π=3.14

In my case I took a string and wrapped it around the center tread of my spare 285/70R17 and marked where the string met.
I laid it out along a tape measure and measured 103"
Dropping it into the above formula I came up with d=103/3.14 which is 32.8".

According to my tire's spec sheets that's pretty hecking close. I think my specs call for a 32.7 and that difference could slight inaccuracies in my measuring device and/or be due to my wheel width vs manufacture wheel width.

I'm curious to see what others come up with using the above method (looking at you, BFGoodrich! :unsure:)
 

jjvw

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If two tire candidates are similar in size but one performs better, which do you choose?
 
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rasband

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I get the feeling this is a reference to something. But if it's not, can you describe this? Like, front to back instead of up and down?
This topic can get contentious for some reason, so it’s a reference to a previously locked thread on the topic. The joke being measuring a circle in any dimension is fine.

Honestly, from a measuring perspective your way seems the most sensible assuming unlaiden and at whatever PSI you run. But at the end of the day, that measurement is mostly irrelevant.

As @jjvw asked - if you could get a “true 35” X or a slightly less than 35 Y that performs better, would the academically accurate size change anything?
 
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jodomcfrodo

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...


I also just buy whatever tire I want in the size I want. If it runs an inch 'small' I really don't care. I've messed around with measuring my tires, but just because I'm curious. If you ever hear me on the trail yell "If these damn BFG's didn't run small I'd have made it", just take my keys and drive away.

That being said, there is a right way to measure a tire dependent on what you are trying to do with that measurement.
 
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samobius

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This topic can get contentious for some reason, so it’s a reference to a previously locked thread on the topic. The joke being measuring a circle in any dimension is fine.

Honestly, from a measuring perspective your way seems the most sensible assuming unlaiden and at whatever PSI you run. But at the end of the day, that measurement is mostly irrelevant.

As @jjvw asked - if you could get a “true 35” X or a slightly less than 35 Y that performs better, would the academically accurate size change anything?
You're right; this is a sensitive topic for some reason which is why I was hoping to bring some serious discussion into it.

When people say the BFGs run small, does that mean their actual circumference isn't what's advertised? Or that their contact area spreads out more, making them appear shorter? I think one matters to the people that are concerned about fender clearance, rubbing, etc and the other matters to the people that want a tire that looks bigger and/or keeps the vehicle further off the ground.
 
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Mr. Bills

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All Posts Matter!!

Including the humorous and semi off topic posts that no one has objected to. Even if those posts are not serious enough for one person's or one moderator's particular tastes.
 
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Chris

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Someone is censoring this thread and not being up front about it.
What's the point of the thread? The guy was asking a serious question. Are you or anyone else contributing at all by being a smart ass? A post or two of banter I get, but this was 2-pages of nonsense and not a single serious response other than what @rasband said.

Once in a while it's funny (I even laugh a lot at some of it), but this is the second thread that's been derailed on this matter. I shut the other one down since it was turning into a shit show / pissing contest.

At what point does the discussion become serious? 2 pages of nonsense posts, how about some actual helpful information for the OP? He was asking a serious question near as I can tell.

I'm merely trying to keep the discussion on-topic and at least somewhat informational, especially after that shit show of a thread from yesterday.
 

Apparition

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What's the point of the thread? The guy was asking a serious question. Are you or anyone else contributing at all by being a smart ass?

I get it, once in a while it's funny (I even laugh a lot at some of it), but this is the second thread that's been derailed on this matter.

At what point does the discussion become serious? 2 pages of nonsense posts, how about some actual helpful information for the OP? He was asking a serious question near as I can tell.
He has a valid way of determining diameter, there are several ways of doing it.

Depending on the goal some ways are more useful.
 

Mr. Bills

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@Chris

If you had bothered to read my first post in this thread that you apparently now admit you censored, you would have seen that i agreed with OP that measuring circumference and dividing by Pi is indeed effective for determining tire diameter.

Its your board and you can do what you want, but censoring content because it isn't "useful enough" for you or forcing a stop to banter between members who are not objecting to it is a slippery slope that can destroy a forum.
 
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rasband

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You're right; this is a sensitive topic for some reason which is why I was hoping to bring some serious discussion into it.

When people say the BFGs run small, does that mean their actual circumference isn't what's advertised? Or that their contact area spreads out more, making them appear shorter? I think one matters to the people that are concerned about fender clearance, rubbing, etc and the other matters to the people that want a tire that looks bigger and/or keeps the vehicle further off the ground.
One way I have heard a lot, but as you noted can be flawed, is measuring the spare's diameter at your desired pressure. However I think the string method really does make the most sense for a quality measurement. Both ways are probably fairly reliable when done properly and measured unlaiden and at a consistent PSI - which IMO is the key to a measurement.

Diameter is only one of many parts at play with whether or not a tire will fit your rig though - so it's a variable to include - but it seems like cycling suspension with tires mounted might be the only way to know with 100% confidence what will and wont fit. You're looking at a combination of: diameter, aspect ratio, tread pattern, (effective) wheel offset (e.g. including spacers), ...
 

Chris

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If you had bothered to read my first post in this thread that you
apparently now admit you censored, you would have seen that i agreed with OP that measuring circumference and dividing by
Pi is indeed effective for determining diameter.
Okay, so it just devolved into a continuation of yesterdays thread after that one post, correct?
 

kmq102

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Guys, why are you all so obsessed with the "size" you are working with? It either gets the job done, or it doesn't. You can say it's 30, 33, or 35 but really as long as there are smiles on faces at the end it is good enough.
 

jodomcfrodo

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What's the point of the thread? The guy was asking a serious question. Are you or anyone else contributing at all by being a smart ass? A post or two of banter I get, but this was 2-pages of nonsense and not a single serious response other than what @rasband said.

Once in a while it's funny (I even laugh a lot at some of it), but this is the second thread that's been derailed on this matter. I shut the other one down since it was turning into a shit show / pissing contest.

At what point does the discussion become serious? 2 pages of nonsense posts, how about some actual helpful information for the OP? He was asking a serious question near as I can tell.

I'm merely trying to keep the discussion on-topic and at least somewhat informational, especially after that shit show of a thread from yesterday.
And down the hill we go.
 

Chris

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I also understand the frustration some new Jeep owners may feel.

If you are new to Jeeps and you buy a tire listed on tire rack as measuring 34.4" (315 size BFG KO2s), put it on your spare tire, and then measure it mounted to your spare and come back with a 33.3" measurement, you might be disappointed.

This if course assumes someone is a new Jeep owner and doesn't know any better.
 
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Chris

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If you’re going to delete posts just delete the thread. It’s clearly a troll thread anyway.
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.
 

billiebob

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Size is overstated, there are lot of factors more important than a 1/4" difference between 2 tires and if the 1/4" means the tire no longer fits..... maybe you are running to close to the margins. But if you want an accurate measure all manufacturers include exact measures in the statistics for every tire they make. I find the most useful measure, beyond actual width, is revolutions per mile.