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Tire size measurement methods


Jerry Bransford

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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."
I wish I had a dollar every time someone disagreed with me here when I said a tire's size can't be determined by measuring its height from the ground to the center of the hub and then doubling it. That has been a popular contention here lately to which I learned I was shouting into the wind when disagreeing with it.
 

Mr. Bills

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jjvw said:
None of the measuring techniques in this thread are the correct measurement for Grimmjeeper. ;)
Sorry, but what is sometimes characterized as the correct tire diameter measurement/calculation technique is too much work. I could measure the rolling circumference with a tape and do some math. I could measure horizontally if I could get the tape in the right place on the crown of the tire. Or I might actually have to leave the house, drive a couple of miles to In-and-Out and correlate my GPS speed with tach rpm while I'm out. But then I'd have to go back home to the gear calculator and work backwards to find the precise tire diameter that nets that speed at that rpm with my known axle gear ratio and transmission. My fries will get cold. 😟

I don't need to be that precise. The only times I have ever even wanted to know the diameter of my tires any more closely than the size molded into the sidewall is to play with gear calculators. For that, I often use the "double the loaded radius" (aka "twice hub height") technique because its easy, quick, with consistent results- even though I know results are not entirely accurate. However, it also only takes two short steps from my chair with a tape measure and usually provides at least as good a guesstimate of cruising rpm with various ratios as using the published tire diameter which I would have to expend effort to google.

When I actually had a legitimate purpose for using a gear calculator, meaning a purpose other than wasting time waiting for USPS to deliver today's Amazon package, the available gear ratio options for my Rubicon Dana 44's coupled with 42RLE and "metric" 35's were 4.88, 5.18 or 5.38. Since I already had 4.88 gears and knew they sucked, my odds of picking the best ratio even without a calculator instantly improved to 50/50. Playing with the gear calculator and published diameters suggested that 5.13 would put my cruising rpms close to where I wanted. Playing with the same calculator and "2x rolling radius" gave higher rpms. The "horizontal" diameter measurement put me somewhere in between. [Mickey Thompson doesn't publish revs/mile so using that value wasn't an option.]

So I did what any rational person would do under the circumstances. I turned the fucking gear calculator off , closed my ears to crowdspeak on the Internet, and went looking for a TJ with 42RLE, 5.13 gears and "metric 35's" to test drive. As luck would have it, the gear shop in the next county south owned a TJ set up exactly that way, and as it turned out the test drive revealed that 5.13 was not enough gear. The "2x rolling radius" tire diameter guesstimate on that particular occasion was a better predictor of the ratio that was best for my jeep as actually set up.

I am too old school, too cheap and too cautious to take a chance buying gears based upon the results of a gear calculator alone. My purpose for the gear calculator required a level of accuracy that would point me in the right direction, little or nothing more. I believe there is no substitute for test driving. However, I understand that sometimes rigs must be built right the first time with little or no opportunity for advance testing or time/money for wrong gear choices and precise gear ratio calculations can be important. If I were in that situation I could probably be persuaded to get off my ass and record some of those speeds and rpms on the way to In-and Out and calculate my tire diameter the "correct" way upon my return home. ;)

Until then, the 2x "loaded radius" aka "hub height" and/or published diameter is close enough for rock and roll (and Internet forums).

:)

But seriously, I really have zero reason to look at a gear calculator at any time in the foreseeable future so if anyone asks the diameter of my Mickey Thompson Baja ATZ-P3 LT315/75/R16 "metric 35" tires, I intend to respond without hesitation, "Thirty-five Fucking Inches of Rotating Goodness."

Who would challenge that? No one has ever asked if my penis is a "true" ten inches.
:eek:
 

jjvw

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I wish I had a dollar every time someone disagreed with me here when I said a tire's size can't be determined by measuring its height from the ground to the center of the hub and then doubling it. That has been a popular contention here lately to which I learned I was shouting into the wind when disagreeing with it.
It depends on what you are wanting to know. 2x hub height is a very real thing and you can verify (or disprove) it anytime you want to. My entire point is to undermine this idea that the "true size" or any published size is the whole story.

At the end of the day and since most here are not in a regulated racing event, a fraction of an inch discrepancy in a published size is largely irrelevant. Once a tire is selected for it's desired performance, it is the actual installed dimensions that matter. Hub heights and axle cycling will tell you what you need to know.
 
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mrblaine

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I wish I had a dollar every time someone disagreed with me here when I said a tire's size can't be determined by measuring its height from the ground to the center of the hub and then doubling it. That has been a popular contention here lately to which I learned I was shouting into the wind when disagreeing with it.
You haven't been able to wrap your head around why that matters yet. When you figure that out, then it makes more sense. I'm not interested in circumference, I'm not interested in spending any time on the manufacturers pages looking up and calculating revolutions per mile. I grow weary of folks ignoring basic math and then using that to dismiss a calculator that will tell us what gear ratio we need or should at least contemplate.

You recommend gear ratios based on tire size and transmission choice, so do I or at least I used to. Now I avoid those discussions as much as possible because I've quit wasting my time explaining the logic only to have it dismissed because they won't use the calculator properly. It is always the same, yeah, I plugged in my MPH, RPM, Trans, and tire size and that piece of shit calculator is way off so I'm not going to use it.

I spent hours explaining that it is just basic math that can't be wrong. We know trans ratios, we know t-case ratios, we know MPH, we know RPM, it can't be wrong. Yes it is, it sucks, I'm just going to go get the 4.88's everyone says are awesome and run them. You do know there is only one variable that affects the outcome, right? No, it's wrong, not gonna use it.

So Jerry, how do I get them to understand why what they see on the dash does NOT match what they plugged into the calculator?

Here is why that matters to me- If someone wants me to assist them in making their dream come true with their concept of a machine that will do everything they want it to, I need them to either trust me 100% or go the fuck away. If you only trust part of what I know, I have zero interest in working with you.

What do I tell them to make them see why their fucking numbers are wrong?

You trust that I can weigh your rig with springs, you trust that I can get the geometry correct, you trust that I can use a tape measure and get both shock towers or coil over hoops the same distance up off the frame and angled the same, you trust that I can dial in a pinion, you trust that I can set the steering angle, toe, caster, etc., but you won't trust a simple tire calculation that helps pick a gear ratio. WHY?
 

mrblaine

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It depends on what you are wanting to know. 2x hub height is a very real thing and you can verify (or disprove) it anytime you want to. My entire point is to undermine this idea that the "true size" or any published size is the whole story.

At the end of the day and since most here are not in a regulated racing event, a fraction of an inch discrepancy in a published size is largely irrelevant. Once a tire is selected for it's desired performance, it is the actual installed dimensions that matter. Hub heights and axle cycling will tell you what you need to know.
When we outboard, we lock the rig down at ride height on jack stands. That is the very first thing we do. The second is we take 2 dimensions. Hub center to ground. Distance between spring perches. If we are moving perches, we get a frame to axle number.

With those 2 numbers, I can cycle, droop, articulate, fold, spindle, mutilate, and manipulate the axle to full stuff, full droop, full articulation both directions and always return right back to ride height to get the correct amount of shock shaft showing that we need.

Not once ever has any tire measured horizontally to be twice what the hub height to ground number is.

What is the most fun about that is when we are trimming blank armor working from ride height and we need the axle at the right height without springs in. We have to air the tire down to single digits so just the weight of the axle will drop it low enough to get the hub height.
 
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Mr. Bills

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What do I tell them to make them see why their fucking numbers are wrong?
Tell them to do today's Saturday morning experiment demonstrating scientific method and practical math:



HOW TO DETERMINE TIRE SIZE FOR USE IN GRIMM JEEPER ONLINE GEAR RATIO CALCULATOR

Today, boys and girls, we are going to use Scientific Method to demonstrate something you can do in real life after the governors let us out of our houses - determine the tire diameter to input into that online gear calculator all the cool jeepers are using so you can figure out which gears to buy with the next government stimulus check.

The first rule of Scientific Method in an experiment is to only change one variable at a time.

Today we are going to use that rule to our advantage by assuming the tire diameter as the unknown variable, plugging in our existing gear ratio, and using the gear calculator to tell us the diameter of our tires rather than which gear ratio to use.

Let's see what the gear calculator needs to give us useful numbers:

1. Transmission type.
2. Transfer case type.
3. Axle ratio
4. Tire diameter.

Numbers 1 and 2 are easy - just go look at your jeep or your build sheet.

Numbers 3 and 4 are where our experiment takes place. The Grimm Jeeper calculator is designed to input a specified tire diameter, standard size or revolutions/mile in order to calculate the unknown variable which for re-gearing purposes is often the rpm at a specified speed and specified gear ratio. The issue of accuracy arises because there is no way to validate published diameters or other tire attributes supplied by third parties with the potential for "garbage in, garbage out" results. However, by using the calculator "backwards," i.e. by supplying a known and verifiable gear ratio rather than a tire diameter, we can figure out the tire diameter we should be using in the calculator for the tires we currently own and use.

This experiment shows you how to use the Grimm Jeeper calculator to tell you the diameter of your tires with amazing precision. You can use that diameter in future calculations to determine the rpms at various speeds with different gear ratios, transmissions, etc.

You will need:

A. A calibrated vehicle speedometer or GPS.
B. A working tachometer
C. A straight stretch of highway where you can safely drive 65 mph or so and record the rpm's in top gear at that speed.

Tip: It is difficult if not impossible to record the exact rpm from a TJ tach unless the needle is dead on one of the marks or numbers. It works just as well to get close to the speed at which you intend to record results, say 65 mph, and then adjust your speed so that the tach needle is directly on a mark, say 2500 rpm, and record the digital mph on the GPS at 2500 rpm rather than the rpm at a predetermined speed.

In my case, after taking several 5-mile runs on a straight, relatively level lightly trafficked Interstate at my usual 26 psi, I determined that in overdrive, with Mickey Thompson Baja ATZ-P3 LT315/75R16 tires, 42RLE transmission, Rubi-Crawler underdrive, 241OR transfer case and 5.38 gears I am travelling "just under" 2500 rpm at exactly 65 mph or exactly 2500 rpm at exactly 66 mph.

I used a Garmin Drive 51 navigation unit and a Garmin inReach Explorer+ to determine speed. I took the opportunity to compare with my TJ speedo. All three matched thanks to my Speedohealer.

Now let's solve for tire diameter.

Enter all of the data required by the online calculator except tire diameter. Go to the bottom left of the calculator where it says "Road RPMs at given speed in miles per hour"and enter the speed at which you conducted the experiment, which in my case was 66 mph.

If you are good at math you can derive the formula for extrapolating tire diameter. I'm not, so I did this instead:

I plugged in 35" for tire diameter because surely Mickey Thompson wouldn't sell undersized tires. Would they?

- the result was 2352 rpm in the bottom left chart at 66 mph. Damn, they can't be 35" because at 66 mph
my rpms were 2500. They must be smaller.

Then I plugged in 34.6", the diameter published by Mickey Thompson and the industry "standard" diameter for 315/75R16 tires.

- the result was 2379 rpm at 66 mph, so they have to be even smaller in effective diameter than 34.6. WTF!

I kept going. Not 34 1/4, not 34, not even 33.5". Those rat bastards at Mickey Thompson.


The final result:

With my equipment as listed, the Grimm Jeeper gear ratio calculator showed that at 66 mph in overdrive at 2499 rpm I had to be travelling on tires with a diameter of . . . . drum roll please . . . . . 32.94".

Call the police!!! I've been robbed! Where's my 2.06"????


Conclusions:

1. The published tire diameter is a bullshit number for purposes of calculating a desired gear ratio. Don't bother to use it.

2. "True" 35" tires are like unicorns, no rational person believes they exist.



Extra Credit:

Just for fun, I compared the 32.94" tire diameter determined by the calculator with other methods.

Hub height aka rolling radius method at 26 psi = 16.25" x 2 = 32.50"

Level placed on top of tire, measured to ground with yardstick at 26 psi = "about" 33".

Measured horizontally at hub, 26 psi with loaded tire on ground (can't measure my spare unloaded because its a different size) = "about" 34".

Length of string around circumference of loaded tire on ground at 26 psi = 107.0" / 3.14 = 34.07"


You can draw your own conclusions from this second set of numbers.

Mine is that the 2x hub height/rolling radius method is probably close enough for rock and roll and forum debates and can point someone in the right direction at least long enough to get out on the road and actually gather data at cruising speed. The other methods not so much. I have not changed my opinion that nothing beats a test drive with the actual gear and tire combination one is considering. [Or if one is a true time management expert, simply write down the number Blaine suggests after you have answered his questions because he has done this long enough with enough different rigs and enough different tires that those numbers aren't coming out of his ass, buy those, and move on to the next conundrum of life.]

And that's the practical science experiment for today.



Future validation of results:

Using 32.94" as the diameter for my existing tires, and with my existing 5.38 gears, I should see the following the next time I take a long trip down I-5:

66 mph - 2499 rpm
70 mph - 2651 rpm
75 mph - 2840 rpm
79 mph - 2991 rpm (the speed over which one is likely to be ticketed on my stretch of I-5.)

We shall see.

____________________________

@mrblaine . You know its your fault that my fries are now cold. ;)
 
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jjvw

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You haven't been able to wrap your head around why that matters yet. When you figure that out, then it makes more sense. I'm not interested in circumference, I'm not interested in spending any time on the manufacturers pages looking up and calculating revolutions per mile. I grow weary of folks ignoring basic math and then using that to dismiss a calculator that will tell us what gear ratio we need or should at least contemplate.

You recommend gear ratios based on tire size and transmission choice, so do I or at least I used to. Now I avoid those discussions as much as possible because I've quit wasting my time explaining the logic only to have it dismissed because they won't use the calculator properly. It is always the same, yeah, I plugged in my MPH, RPM, Trans, and tire size and that piece of shit calculator is way off so I'm not going to use it.

I spent hours explaining that it is just basic math that can't be wrong. We know trans ratios, we know t-case ratios, we know MPH, we know RPM, it can't be wrong. Yes it is, it sucks, I'm just going to go get the 4.88's everyone says are awesome and run them. You do know there is only one variable that affects the outcome, right? No, it's wrong, not gonna use it.

So Jerry, how do I get them to understand why what they see on the dash does NOT match what they plugged into the calculator?

Here is why that matters to me- If someone wants me to assist them in making their dream come true with their concept of a machine that will do everything they want it to, I need them to either trust me 100% or go the fuck away. If you only trust part of what I know, I have zero interest in working with you.

What do I tell them to make them see why their fucking numbers are wrong?

You trust that I can weigh your rig with springs, you trust that I can get the geometry correct, you trust that I can use a tape measure and get both shock towers or coil over hoops the same distance up off the frame and angled the same, you trust that I can dial in a pinion, you trust that I can set the steering angle, toe, caster, etc., but you won't trust a simple tire calculation that helps pick a gear ratio. WHY?
Hopefully you aren't shouting into the wind this time.
 

jjvw

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....

Tip: It is difficult if not impossible to record the exact rpm from a TJ tach unless the needle is dead on one of the marks or numbers. It works just as well to get close to the speed at which you intend to record results, say 65 mph, and then adjust your speed so that the tach needle is directly on a mark, say 2500 rpm, and record the digital mph on the GPS at 2500 rpm rather than the rpm at a predetermined speed.

...
The RPMs can be read directly from the PCM via ODBII. Do this with a phone and you can have a pair of dials showing engine RPM and GPS speed next to each other.
 
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Zorba

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It depends on what you are wanting to know. 2x hub height is a very real thing and you can verify (or disprove) it anytime you want to. My entire point is to undermine this idea that the "true size" or any published size is the whole story.

At the end of the day and since most here are not in a regulated racing event, a fraction of an inch discrepancy in a published size is largely irrelevant. Once a tire is selected for it's desired performance, it is the actual installed dimensions that matter. Hub heights and axle cycling will tell you what you need to know.
I've been watching this argument go on, and on, and on. FWIW, here's my takeaway:

1) Measuring the circumference and doing the math will give the "true" diameter - but that's pretty much useless...
2) Measuring horizontally - or even diagonally, *can* get you close to "true" depending on how/where you eyeball things - but that measurement is still useless.
3) Measuring vertically - with load - is completely useless. Measuring the spare vertically will get you the same results as #2.
4) 2X hub height (with load), at your desired air pressure, will give you the EFFECTIVE diameter. This is the ONLY measurement that matters or means anything. Similar results should be obtainable with RPM vs Speed but is a nuisance to measure accurately.
 

BuildBreakRepeat

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Tell them to do today's Saturday morning experiment demonstrating scientific method and practical math:



HOW TO DETERMINE TIRE SIZE FOR USE IN GRIMM JEEPER ONLINE GEAR RATIO CALCULATOR

Today, boys and girls, we are going to use Scientific Method to demonstrate something you can do in real life after the governors let us out of our houses - determine the tire diameter to input into that online gear calculator all the cool jeepers are using so you can figure out which gears to buy with the next government stimulus check.

The first rule of Scientific Method in an experiment is to only change one variable at a time.

Today we are going to use that rule to our advantage by assuming the tire diameter as the unknown variable, plugging in our existing gear ratio, and using the gear calculator to tell us the diameter of our tires rather than which gear ratio to use.

Let's see what the gear calculator needs to give us useful numbers:

1. Transmission type.
2. Transfer case type.
3. Axle ratio
4. Tire diameter.

Numbers 1 and 2 are easy - just go look at your jeep or your build sheet.

Numbers 3 and 4 are where our experiment takes place. The Grimm Jeeper calculator is designed to input a specified tire diameter, standard size or revolutions/mile in order to calculate the unknown variable which for re-gearing purposes is often the rpm at a specified speed and specified gear ratio. The issue of accuracy arises because there is no way to validate published diameters or other tire attributes supplied by third parties with the potential for "garbage in, garbage out" results. However, by using the calculator "backwards," i.e. by supplying a known and verifiable gear ratio rather than a tire diameter, we can figure out the tire diameter we should be using in the calculator for the tires we currently own and use.

This experiment shows you how to use the Grimm Jeeper calculator to tell you the diameter of your tires with amazing precision. You can use that diameter in future calculations to determine the rpms at various speeds with different gear ratios, transmissions, etc.

You will need:

A. A calibrated vehicle speedometer or GPS.
B. A working tachometer
C. A straight stretch of highway where you can safely drive 65 mph or so and record the rpm's in top gear at that speed.

Tip: It is difficult if not impossible to record the exact rpm from a TJ tach unless the needle is dead on one of the marks or numbers. It works just as well to get close to the speed at which you intend to record results, say 65 mph, and then adjust your speed so that the tach needle is directly on a mark, say 2500 rpm, and record the digital mph on the GPS at 2500 rpm rather than the rpm at a predetermined speed.

In my case, after taking several 5-mile runs on a straight, relatively level lightly trafficked Interstate at my usual 26 psi, I determined that in overdrive, with Mickey Thompson Baja ATZ-P3 LT315/75R16 tires, 42RLE transmission, Rubi-Crawler underdrive, 241OR transfer case and 5.38 gears I am travelling "just under" 2500 rpm at exactly 65 mph or exactly 2500 rpm at exactly 66 mph.

I used a Garmin Drive 51 navigation unit and a Garmin inReach Explorer+ to determine speed. I took the opportunity to compare with my TJ speedo. All three matched thanks to my Speedohealer.

Now let's solve for tire diameter.

Enter all of the data required by the online calculator except tire diameter. Go to the bottom left of the calculator where it says "Road RPMs at given speed in miles per hour"and enter the speed at which you conducted the experiment, which in my case was 66 mph.

If you are good at math you can derive the formula for extrapolating tire diameter. I'm not, so I did this instead:

I plugged in 35" for tire diameter because surely Mickey Thompson wouldn't sell undersized tires. Would they?

- the result was 2352 rpm in the bottom left chart at 66 mph. Damn, they can't be 35" because at 66 mph
my rpms were 2500. They must be smaller.

Then I plugged in 34.6", the diameter published by Mickey Thompson and the industry "standard" diameter for 315/75R16 tires.

- the result was 2379 rpm at 66 mph, so they have to be even smaller in effective diameter than 34.6. WTF!

I kept going. Not 34 1/4, not 34, not even 33.5". Those rat bastards at Mickey Thompson.


The final result:

With my equipment as listed, the Grimm Jeeper gear ratio calculator showed that at 66 mph in overdrive at 2499 rpm I had to be travelling on tires with a diameter of . . . . drum roll please . . . . . 32.94".

Call the police!!! I've been robbed! Where's my 2.06"????


Conclusions:

1. The published tire diameter is a bullshit number for purposes of calculating a desired gear ratio. Don't bother to use it.

2. "True" 35" tires are like unicorns, no rational person believes they exist.



Extra Credit:

Just for fun, I compared the 32.94" tire diameter determined by the calculator with other methods.

Hub height aka rolling radius method at 26 psi = 16.25" x 2 = 32.50"

Level placed on top of tire, measured to ground with yardstick at 26 psi = "about" 33".

Measured horizontally at hub, 26 psi with loaded tire on ground (can't measure my spare unloaded because its a different size) = "about" 34".

Length of string around circumference of loaded tire on ground at 26 psi = 107.0" / 3.14 = 34.07"


You can draw your own conclusions from this second set of numbers.

Mine is that the 2x hub height/rolling radius method is probably close enough for rock and roll and forum debates and can point someone in the right direction at least long enough to get out on the road and actually gather data at cruising speed. The other methods not so much. I have not changed my opinion that nothing beats a test drive with the actual gear and tire combination one is considering. [Or if one is a true time management expert, simply write down the number Blaine suggests after you have answered his questions because he has done this long enough with enough different rigs and enough different tires that those numbers aren't coming out of his ass, buy those, and move on to the next conundrum of life.]

And that's the practical science experiment for today.



Future validation of results:

Using 32.94" as the diameter for my existing tires, and with my existing 5.38 gears, I should see the following the next time I take a long trip down I-5:

66 mph - 2499 rpm
70 mph - 2651 rpm
75 mph - 2840 rpm
79 mph - 2991 rpm (the speed over which one is likely to be ticketed on my stretch of I-5.)

We shall see.

____________________________

@mrblaine . You know its your fault that my fries are now cold. ;)
Excellent write up. That is exactly how I calculated mine when researching gears.
 

Mr. Bills

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. . . 4) 2X hub height (with load), at your desired air pressure, will give you the EFFECTIVE diameter. This is the ONLY measurement that matters or means anything. Similar results should be obtainable with RPM vs Speed but is a nuisance to measure accurately.
It wasn't a nuisance at all. I drove to In-and-Out.
 
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Jerry Bransford

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I've been watching this argument go on, and on, and on. FWIW, here's my takeaway:
4) 2X hub height (with load), at your desired air pressure, will give you the EFFECTIVE diameter.
I would disagree with that one. Remember where Blaine said...
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."
The measured height can be nearly anything you want it to be, just vary the air pressure, and it will have nothing to do with the tire's real size.
 

Zorba

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I would disagree with that one. Remember where Blaine said...
The measured height can be nearly anything you want it to be, just vary the air pressure, and it will have nothing to do with the tire's real size.
That is absolutely correct. That is why I said, and I repeat for emphasis, EFFECTIVE size. And yes, it absolutely will vary by air pressure. Its what you'll really have rolling down the road. "Real" or "true" size means little other than for comparison purposes. Effective size is what you want to use for any calculations - gear size or otherwise - that you may want to perform.
 

Abell02TJ

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Rim size + 2x sidewall height...? I’ve never even given it a first thought let alone a second until now. My 275/60R20s would be 32” by that method and are 32” by my tape measure so good enough for me.
 

jjvw

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Rim size + 2x sidewall height...? I’ve never even given it a first thought let alone a second until now. My 275/60R20s would be 32” by that method and are 32” by my tape measure so good enough for me.
Why bring rim size and sidewall height into the confusion?