How to measure your TRUE tire size! - JKowners.com : Jeep Wrangler JK Forum
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post #1 of 26 Old 11-23-2012, 01:56 PM Thread Starter
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How to measure your TRUE tire size!

Hopefully someone will sticky this, for the sake of our collective sanity...

Alot of people ('these days') seem to be measuring their tires and then complaining that they are not true to size... eg. my 35" tires only measure 33.5"

SO, before you start complaining about this, please, please, please measure your tires the right way... this means mounted on your rim, aired up, and UNLOADED (ie. not on your Jeep!).

If you measure the height of your tire when it is on your Jeep and loaded, you can be guaranteed that it will measure small. Why?? Because tire sizes are not specified that way! You have to measure the circumference (and calculate the diameter) with the tire mounted, and aired up, and un-loaded. Yes, good idea is to measure your spare if you can (run a line or tape around the centre of the tire to measure circumference, and then calculate the diameter).

Diamater = circumference / pi

(so take your circumference and divide by 3.14 to get your true diameter)

Measuring from side-side whilst tire is still on the Jeep (and under load) would work OK if you are accurate enough (with where you hold the tape). In fact, many tire manufacturers specify the tire diameter as: "Diameter: The measurement of an unloaded tire from one outer edge to the opposite edge." But not as accurate as the circumference measurement method.

Even if you are looking at section height you have to do it unloaded: "Section Height: The vertical distance measured from the edge of the bead to the center of the crown in a tire that is not under load."

The other way to measure cirumference whilst still on the car is to mark the tire and ground with chalk at the bottom of the tire, and then roll forward until the mark is at the bottom again and mark the ground there - measure the distance between the 2 lines, and divide by 3.14 to get your diameter.

...and just another example of how tire sizes should be measured, from MT:
-----------------------------------------

Overall Diameter (O.D.)

O.D. is the measurement of a tire diameter and is taken from the top of the tire to the bottom of the tire. Note: The O.D. of a tire is measured with the tire off of the vehicle, not under load and at appropriate air pressure. The most accurate way to calculate the OD of a tire is measuring the circumference and dividing by 3.1416”. (Example: Circumference = 88.0” ; therefore 88.0 divided by 3.1416 = 28.01”) Knowing the O.D. of a tire will aid in fitment issues and can assist in gear ratio and speedometer corrections.

All Mickey Thompson® tires are listed in our specification sheets with an O.D. (overall diameter) or circumference. These measurements are derived in the following manner . . .
-------------------------------------------------

Please stop measuring tires mounted and loaded, and then complaining that they are not true to size... measure them the right way, then complain your hearts out!

S.

From the land downunder...

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post #2 of 26 Old 11-23-2012, 02:34 PM
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unless youre measuring them for the purpose of setting your speedometer via programmer - procal etc
then you need to measure them at "ride height" with the vehicle sitting on them since thats the condition theyll be in when the computer is converting wheel revolutions to MPH


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post #3 of 26 Old 11-23-2012, 04:20 PM Thread Starter
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Yep - follow the instructions to set the procal... I think it says to measure from hub to ground and then double it.

S.

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post #4 of 26 Old 11-23-2012, 04:29 PM
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I put a piece of chalk in the tread and moved the Jeep until there were two chalk marks on the pavement.
Measured between them and divided by 3.1416.

That's pretty accurate, too.

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post #5 of 26 Old 12-06-2012, 05:50 PM
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Quote:
Originally Posted by venom View Post
unless youre measuring them for the purpose of setting your speedometer via programmer - procal etc
then you need to measure them at "ride height" with the vehicle sitting on them since thats the condition theyll be in when the computer is converting wheel revolutions to MPH
... Which is why we bitch about true size.

Man... you must work for BFG.
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post #6 of 26 Old 12-06-2012, 05:59 PM
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... Which is why we bitch about true size.

Man... you must work for BFG.
lol! Now that's funny chit right there!
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post #7 of 26 Old 12-07-2012, 06:58 AM Thread Starter
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You guys crack me up. Ignorance is bliss huh?? No worries, go back to complaining about anything and everything... Next time maybe we can have a spelling contest... Hmmm...

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post #8 of 26 Old 12-07-2012, 02:48 PM
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Well... honestly,
our only real complaint is about false advertising.
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post #9 of 26 Old 12-07-2012, 03:39 PM
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Well... honestly,
our only real complaint is about false advertising.
Agreed! When no matter how you measure it and it's still almost 2"s shy of advertised, that's fucked up!
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post #10 of 26 Old 12-07-2012, 09:18 PM Thread Starter
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2" eh? Measuring the industry standard (correct) way using the circumference, on the correct size rim, at the correct pressure, unloaded??

I agree - that is quite bad. Or your tape measure needs to be re-calibrated... one of the two.

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post #11 of 26 Old 12-08-2012, 02:44 AM
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Originally Posted by FireTruck View Post
2" eh? Measuring the industry standard (correct) way using the circumference, on the correct size rim, at the correct pressure, unloaded??

I agree - that is quite bad. Or your tape measure needs to be re-calibrated... one of the two.
Oh I'm happy with my 37 Trail Grapplers. Other brands, no so much especially thise with BFG's etc! My tape works perfectly fine! Lol
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post #12 of 26 Old 12-09-2012, 04:59 PM
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Happy as well with my Mickey's size.

Dude ... I'm tellin' ya, he works for BFG.

.

.


Sorry, I had to do it.

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post #13 of 26 Old 12-09-2012, 07:05 PM
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Are BFG normally smaller than advertised? Please forgive me if this is a dumb question.
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post #14 of 26 Old 12-09-2012, 07:07 PM
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A lot! And notorious.
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post #15 of 26 Old 12-09-2012, 07:27 PM
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Originally Posted by FireTrail741 View Post
A lot! And notorious.
So if im looking at a 35" BFG KM2 and a 35" GY MTR the Goodyear will be bigger?

I will measure my Nittos and see how they measure up.
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post #16 of 26 Old 12-09-2012, 07:30 PM
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Goodyears are pretty true atleast the 37's. My Nitto trail Grapplers measured something pretty close to actual 37.
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post #17 of 26 Old 12-09-2012, 07:32 PM
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Go look at tires actually mounted and see for yourself. I wanted a true 37.
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post #18 of 26 Old 12-09-2012, 09:30 PM Thread Starter
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I wanted a small 35... So ended up with Mickey MTZ's. I certainly don't work for BFG! I was sponsored by Maxxis - and would run them on my JKU no worries if they made the right size!

Moral of the story is do your research and choose what you want/need. Plenty of info out there, even from the manufacturers themselves.

S.

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post #19 of 26 Old 12-09-2012, 09:44 PM
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Shouldn't it be circumference divided by pie or some shit? I need to draw a picture something. If your tire measures 35 and not mounted with weight, when you add weight you still have the same amount of tire to turn over.

If there's not weight on it, and loaded with X psi of air, it would travel xx inches to make a complete rotation. It would travel the same distance if it has vehicle weight on it.


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post #20 of 26 Old 12-09-2012, 10:56 PM
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Damnit. I was on my phone earlier and i just now read the first post. I guess it pays to read from the beginning.


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post #21 of 26 Old 12-31-2012, 09:48 PM
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Lol funny post. The only manufacturer that I have come across that posts an actual true tire size is Pitbull. 35x12.50x17 rockers are 35.2" actual and will not fit on the factory carrier with the factory bumper.
35" GY Duratracs just shy of 33.75" will fit on the carrier with factory bumper and so will ProComp Xtreme M/T 33.5" and Nitto Mud Grapplers 34".

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post #22 of 26 Old 12-31-2012, 10:28 PM
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My 35" Trail Grapplers are only 33.5 now.... :*(
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post #23 of 26 Old 01-01-2013, 09:09 PM Thread Starter
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... I measured my (new) Mickey Thompson 315/70/17's (which are listed as being 34.6")... and they measured out to 34.55"... yep, rounds up to 34.6", so they are exaclty as in the spec sheet.

(mounted on a 9" wide rim, at 32psi, and measured by circumference and unloaded!)

Guess I got lucky...

S.

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post #24 of 26 Old 01-01-2013, 09:16 PM
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My Pitbulls measured 41" on the ground, mounted, aired up, not installed on the JK. Once I put them on the JK, they're at about 39.75" or so when measured correctly (i.e. ground to center of hub). With 39.75" programmed in via the TrailDash, my speedo is dead balls accurate.


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post #25 of 26 Old 01-02-2013, 11:31 PM Thread Starter
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...yea, to get my speedo nuts on correct I had to use 33.5 on the Procal!

(but my speedo was a few % off from the factory... so with the Procal I could correct it)

S.

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