Tire Size vs. True Size (3 Viewers)

[TeCKis300]

There's been some side discussions on manufacturers size vs true tire size. Let's discuss this as it's been a point of confusion.

A lot of people seem to be measuring their tires and complaining that they are coming up short, e.g. my 35" tires are only 33.5". Why is that? Are the manufacturers looking to pull a fast one? Are they wrong?

The proper way to measure is mounted, aired up, and unloaded! This is the measurement you're getting from manufacturers specs. Measuring any other way, especially on vehicle is a sure fire way to come up short and disappointed. It doesn't mean the manufactures measurement is inaccurate, just because you decided on a "better" methodology. It's just different, and I'll add, not very accurate to measure on car.

Tires are generally not measured on car because it is then subject to variables as load and air pressure, etc. We all know what happens to ground clearance when we air down tires for traction. Also consider that some of the armored cars here weigh almost 8k lbs! Well, what if I measure side to side? Wrong again. Tire carcasses are defined by steel belts. Steel belts don't do much stretching, but can bend. Take a circle, and bend it across one axis. It will grow in the other axis! Not very consistent measurements, subject to load, air pressure, geometry etc.

Other than measuring diameter unloaded, the only other solid way is to measure by circumference (steel belts don't appreciably stretch). Mark the tire and ground, and ground again when the tire completes 1 full revolution. Measure the distance between the 2 lines, and divide by 3.14 to get diameter.


Per Tire Rack:
"Overall Diameter"
A tire's overall diameter is the outer diameter of the tire measured in the center of the tread. This measurement is made without any load placed upon the tire and after the tire has been properly mounted on its industry assigned measuring rim and has been inflated and reset to its test pressure after 24 hours."

Per 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 .


So stop complaining the manufacturers are wrong. Now complain your hearts out.

 

[Markuson]

There's been some side discussions on manufacturers size vs true tire size. Let's discuss this as it's been a point of confusion.

A lot of people seem to be measuring their tires and complaining that they are coming up short, e.g. my 35" tires are only 33.5". Why is that? Are the manufacturers looking to pull a fast one? Are they wrong?

The proper way to measure is mounted, aired up, and unloaded! This is the measurement you're getting from manufacturers specs. Measuring any other way, especially on vehicle is a sure fire way to come up short and disappointed. It doesn't mean the manufactures measurement is inaccurate, just because you decided on a "better" methodology. It's just different, and I'll add, not very accurate to measure on car.

Tires are generally not measured on car because it is then subject to variables as load and air pressure, etc. We all know what happens to ground clearance when we air down tires for traction. Also consider that some of the armored cars here weigh almost 8k lbs! Well, what if I measure side to side? Wrong again. Tire carcasses are defined by steel belts. Steel belts don't do much stretching, but can bend. Take a circle, and bend it across one axis. It will grow in the other axis! Not very consistent measurements, subject to load, air pressure, geometry etc.

Other than measuring diameter unloaded, the only other solid way is to measure by circumference (steel belts don't appreciably stretch). Mark the tire and ground, and ground again when the tire completes 1 full revolution. Measure the distance between the 2 lines, and divide by 3.14 to get diameter.


Per Tire Rack:
"Overall Diameter"
A tire's overall diameter is the outer diameter of the tire measured in the center of the tread. This measurement is made without any load placed upon the tire and after the tire has been properly mounted on its industry assigned measuring rim and has been inflated and reset to its test pressure after 24 hours."

Per 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 .


So stop complaining the manufacturers are wrong. Now complain your hearts out.

If you are a tire company(?), I can understand that you might get accused of this or that, but I really don't hear complaints that the manufacturers are trying to fool anyone.
Where are you hearing that?

It's just that it's very hard to know what you'll end up with prior to purchase/installation, and that gets discussed.

An easier way of saying that is...
...Manufacturers specs are TECHNICALLY accurate...but PRACTICALLY confusing.

I do appreciate your clear understanding and explanation...
...but lets be real for a second: None of us really care how big tires are when they are unmounted.

Now the $64 question is... Will you complain about this clarification? hehe

 

[Taco2Cruiser]

There's been some side discussions on manufacturers size vs true tire size. Let's discuss this as it's been a point of confusion.

A lot of people seem to be measuring their tires and complaining that they are coming up short, e.g. my 35" tires are only 33.5". Why is that? Are the manufacturers looking to pull a fast one? Are they wrong?

The proper way to measure is mounted, aired up, and unloaded! This is the measurement you're getting from manufacturers specs. Measuring any other way, especially on vehicle is a sure fire way to come up short and disappointed. It doesn't mean the manufactures measurement is inaccurate, just because you decided on a "better" methodology. It's just different, and I'll add, not very accurate to measure on car.

Tires are generally not measured on car because it is then subject to variables as load and air pressure, etc. We all know what happens to ground clearance when we air down tires for traction. Also consider that some of the armored cars here weigh almost 8k lbs! Well, what if I measure side to side? Wrong again. Tire carcasses are defined by steel belts. Steel belts don't do much stretching, but can bend. Take a circle, and bend it across one axis. It will grow in the other axis! Not very consistent measurements, subject to load, air pressure, geometry etc.

Other than measuring diameter unloaded, the only other solid way is to measure by circumference (steel belts don't appreciably stretch). Mark the tire and ground, and ground again when the tire completes 1 full revolution. Measure the distance between the 2 lines, and divide by 3.14 to get diameter.


Per Tire Rack:
"Overall Diameter"
A tire's overall diameter is the outer diameter of the tire measured in the center of the tread. This measurement is made without any load placed upon the tire and after the tire has been properly mounted on its industry assigned measuring rim and has been inflated and reset to its test pressure after 24 hours."

Per 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 .


So stop complaining the manufacturers are wrong. Now complain your hearts out.

Yep. Couldn't of said it better myself.

 

[Sandroad]

I do have two technical followup questions.

Most (all?) tire manufacturers provide a range of wheel widths acceptable for a particular tire. Does the width of the wheel a tire is mounted on affect the diameter? If so, that may be relevant to those who are using aftermarket wheels on their 200s that are other than OEM width.

Do the steel belts in a tire run parallel or perpendicular to the circumference? If they run parallel and steel can't stretch much, then the tire should have about the same diameter mounted/inflated as unmounted/uninflated. If they run perpendicular (radially), then it does seem like stretching could occur when mounted/inflated. And, in that case, changes in psi could change diameter (though likely not much).

 

[Markuson]

Not only that, but different tires stretch in different directions when inflated sometimes, too. Recently, someone posted how their two sets of tires behave differently--with one getting wider when inflated, and the other getting taller when inflated. Again, this is why I'm always interested in people's on-200 measurements.

 

[mcgaskins]

There's been some side discussions on manufacturers size vs true tire size. Let's discuss this as it's been a point of confusion.

A lot of people seem to be measuring their tires and complaining that they are coming up short, e.g. my 35" tires are only 33.5". Why is that? Are the manufacturers looking to pull a fast one? Are they wrong?

The proper way to measure is mounted, aired up, and unloaded! This is the measurement you're getting from manufacturers specs. Measuring any other way, especially on vehicle is a sure fire way to come up short and disappointed. It doesn't mean the manufactures measurement is inaccurate, just because you decided on a "better" methodology. It's just different, and I'll add, not very accurate to measure on car.

Tires are generally not measured on car because it is then subject to variables as load and air pressure, etc. We all know what happens to ground clearance when we air down tires for traction. Also consider that some of the armored cars here weigh almost 8k lbs! Well, what if I measure side to side? Wrong again. Tire carcasses are defined by steel belts. Steel belts don't do much stretching, but can bend. Take a circle, and bend it across one axis. It will grow in the other axis! Not very consistent measurements, subject to load, air pressure, geometry etc.

Other than measuring diameter unloaded, the only other solid way is to measure by circumference (steel belts don't appreciably stretch). Mark the tire and ground, and ground again when the tire completes 1 full revolution. Measure the distance between the 2 lines, and divide by 3.14 to get diameter.


Per Tire Rack:
"Overall Diameter"
A tire's overall diameter is the outer diameter of the tire measured in the center of the tread. This measurement is made without any load placed upon the tire and after the tire has been properly mounted on its industry assigned measuring rim and has been inflated and reset to its test pressure after 24 hours."

Per 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 .


So stop complaining the manufacturers are wrong. Now complain your hearts out.

Regardless of the technical explanation, some tires and some manufacturers are FAR off from what they purport - period. My 37" KO2s measured 35.5" - mounted, on a spare rim, on the back of my Jeep, fully aired up, with no load. Another guy I wheeled with at the time had 34" KO2s which measured 32.2" mounted, on a spare rim, on the back of his Jeep, fully aired up, with no load. I also had a set of 33" Super Swampers that measured nearly 34", so again what manufacturers claim doesn't necessarily mean it is so. I think people get frustrated when they plan a build around certain measurements, and when the tire is drastically different from manufacturer to manufacturer it's tough to know what to do. It's widely known that some tires like Nitto measure up much more closely to claims than BFG. In summation, yes some manufacturers are wrong, and I'm not complaining my heart out I'm relaying what I know from direct experience and fact.

My last vehicle with brand new 37" KO2s...that measured 35.5"...mounted, off the vehicle, not under load, at appropriate psi:

 

[TeCKis300]

I do have two technical followup questions.

Most (all?) tire manufacturers provide a range of wheel widths acceptable for a particular tire. Does the width of the wheel a tire is mounted on affect the diameter? If so, that may be relevant to those who are using aftermarket wheels on their 200s that are other than OEM width.

Do the steel belts in a tire run parallel or perpendicular to the circumference? If they run parallel and steel can't stretch much, then the tire should have about the same diameter mounted/inflated as unmounted/uninflated. If they run perpendicular (radially), then it does seem like stretching could occur when mounted/inflated. And, in that case, changes in psi could change diameter (though likely not much).

Great question. Rim width doesn't appreciably affect diameter/circumference. Nor tread width. It primarily affects the sidewall geometry, whether it's square, or more trapezoid in cross section. Where this really has an effect is handling of the vehicle. But for trucks, it's not much of a concern and rarely talked about. You're right that pressure may have a slight affect on overall diameter even unloaded, so there is industry agreed to test pressures as noted by Tire Rack.

 

[TeCKis300]

Regardless of the technical explanation, some tires and some manufacturers are FAR off from what they purport - period. My 37" KO2s measured 35.5" - mounted, on a spare rim, on the back of my Jeep, fully aired up, with no load. Another guy I wheeled with at the time had 34" KO2s which measured 32.2" mounted, on a spare rim, on the back of his Jeep, fully aired up, with no load. I also had a set of 33" Super Swampers that measured nearly 34", so again what manufacturers claim doesn't necessarily mean it is so. I think people get frustrated when they plan a build around certain measurements, and when the tire is drastically different from manufacturer to manufacturer it's tough to know what to do. It's widely known that some tires like Nitto measure up much more closely to claims than BFG. In summation, yes some manufacturers are wrong, and I'm not complaining my heart out I'm relaying what I know from direct experience and fact.

My last vehicle with brand new 37" KO2s...that measured 35.5"...mounted, off the vehicle, not under load, at appropriate psi:

Great point, and I agree. Yet it's another distinct point of confusion. The manufacture sizes I quoted earlier were measured tire spec sizes. These are the manufacturers reported dimensions based on industry agreed upon measuring methodologies.

In your case, you're talking about the differences between Tire inch sizing (i.e. class of tires denoted by 35x12.5) vs measured spec. There indeed have pretty dramatic differences here as you point out. So it's definitely worthwhile to reference measured spec sizing as a cross check.

To a lesser degree, this is true for regular tire class sizing (i.e. 305/55/20). It's worthwhile to always cross reference the measure spec for whatever tire size you think you're getting.

So what's the point in all this. It's the ability to compare apples to apples across manufacturers. If I know I have a tire mounted that is 33.2" based on the manufactures spec and I have another .2" of clearance, I can trust to some degree, that a tire from another manufacturer measuring 33.4 may just fit.

Great looking rig btw.

 

[Taco2Cruiser]

Figured I’d restart this thread with an actually tape measurements of some tires.

BFG KO2 34x10.50r17; 17x8 rim; front axle load: 32.1” tall
Cooper STT Pro 295/70r17; 17x8; front axle load: 32” tall

 

[BW200]

Interesting - both MFG's list the overall diameter as 33.5" for these...

I guess this is a demonstration of the fact that a guy planning his application should check the MFG's specs before he assumes the tire's "size" means it's the actual measurement.

@Taco2Cruiser , what diameter do you measure without the front axle load?

 

[SWUtah]

Regardless of the technical explanation, some tires and some manufacturers are FAR off from what they purport - period. My 37" KO2s measured 35.5" - mounted, on a spare rim, on the back of my Jeep, fully aired up, with no load. Another guy I wheeled with at the time had 34" KO2s which measured 32.2" mounted, on a spare rim, on the back of his Jeep, fully aired up, with no load. I also had a set of 33" Super Swampers that measured nearly 34", so again what manufacturers claim doesn't necessarily mean it is so. I think people get frustrated when they plan a build around certain measurements, and when the tire is drastically different from manufacturer to manufacturer it's tough to know what to do. It's widely known that some tires like Nitto measure up much more closely to claims than BFG. In summation, yes some manufacturers are wrong, and I'm not complaining my heart out I'm relaying what I know from direct experience and fact.

My last vehicle with brand new 37" KO2s...that measured 35.5"...mounted, off the vehicle, not under load, at appropriate psi:

Maybe rim width caused the variation?

 

[pribilof]

I agree that rim width may have some impact, especially with large sidewall ratio tires. Tire manufacturers tell you the rim width the tire specs are based on. Tire sidewall is the hypotenuse of a triangle, granted it's a squishy triangle.

 

[TeCKis300]

Read post #7. No, rim width does not appreciably effect diameter. It really only changes the sidewall profile. And perhaps at the corner where the sidewall meets the tread.

Diameter is dictated by the internal steel belts that run on the face of tread. These run circumferential to the tire and are not in the sidewall. The sidewall is composed of the radial plies that can flex to meet the rim lip. Ever see stanced stretched wide rimmed street cars? I've played with my fair share of them. Eoesn't matter how much or little stretch there is. The tire diameter stays the same.

This might help: