Tire size measurement methods

The best way to measure a tire is to look up the revolutions per mile in the tire specs. Take 20168/revs per mile=tire diameter in inches.

No other way to get a repeatable measurement due to load, inflation pressure, rim width or operator bias
 
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The easiest direct method to measure a tire in my opinion is this. Take a piece of chalk and clearly mark where the tire is touching the ground in a single spot on the outside edge, mark a line that clearly marks the tire and ground that is about 1/4" wide. If you need help finding the center just hang a string roughly where your center cap is and mark down from there. Move the Jeep forward one revolution and put the chalk mark exactly down on the ground again. Use the string if you have trouble eyeballing it. Carefully measure from the front of each chalk mark and divide this number by 3.14. This will give you the exact rolling diameter or the number that is most useful on grimjeeper. I think it's the best way to take a direct measurement and I don't think a free diameter is of much use for anything. Since you are dividing by 3 your error is also going to be pretty small unless you are careless with the tape measure. The flex in the tire will also help allot to eliminate errors.

If you are really nuts about it and used painters tape you could probably get within 1/32" of the rolling diameter.
 
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Chris said:
Isn’t this basically the same as measuring them while mounted on the spare tire carrier? The only difference being that they aren’t on the ground, but I’m not sure how much of a difference that would make in terms of measurement.
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Yes, and same as measuring with a level like posted just before my response. Just offering another option for an 100% accurate measurement
 
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jjvw said:
Once we know this true tire size, what do we do with that knowledge?
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I guess you can plug it into grimm jeeper and see how well calibrated all your gauges are? I was just trying to answer the question without getting too much into it, you know I'm no stickler for exact tire sizes. Still no reason not to just give the simple answer I think.
 
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Vinman said:
Yes, and same as measuring with a level like posted just before my response. Just offering another option for an 100% accurate measurement
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Gotcha! I wasn’t being argumentative, I was just curious if I was missing something.
 
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pagrey said:
I guess you can plug it into grimm jeeper and see how well calibrated all your gauges are? I was just trying to answer the question without getting too much into it, you know I'm no stickler for exact tire sizes. Still no reason not to just give the simple answer I think.
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None of the measuring techniques in this thread are the correct measurement for Grimmjeeper. ;)
 
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If anybody is interested I did a write up that allows you to track your tire diameter very precisely.

How-to calibrate your odometer using OBD, GPS and a SpeedoHealer

I'll be honest, it's not easy math to understand but it's fun if you like that sort of stuff. You can test how Jeep programmed your odometer and track your tire wear and pressure to ridiculously accurate numbers.
 
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pagrey said:
Do whatever you want (sorry, that sounds harsh and wasn't really my point, I mean that works great too)
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One way is accurate for the gear ratio calculators, the others presented here are not. You or anyone else can demonstrate this by entering a speed as read by GPS with the engine rpm into a gear ratio calculator. Change the tire diameter until everything matches up. The resulting tire diameter you enter will be twice the hub height.

Do that and you will discover one of the more meaningful ways to measure a tire.
 
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jjvw said:
Once we know this true tire size, what do we do with that knowledge?
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Use it to answer people’s questions about what runs big, small, “true”, etc. Really just a way to compile a database of information to answer a question that can create a 100 page thread if allowed.
 
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jjvw said:
Do that and you will discover one of the more meaningful ways to measure a tire.
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I think, the linear measurement on the ground is the same as your tire height measurement at the hub, the math is identical. Obviously the free diameters aren't the same. The measurement on the ground is in fact the revolutions per mile in the grim jeeper calculator and you can go between your diameter and the ground measurement with no change in error within your ability to measure a few feet.

Rubicon John said:
Use it to answer people’s questions about what runs big, small, “true”, etc. Really just a way to compile a database of information to answer a question that can create a 100 page thread if allowed.
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Why participate in a discussion if you aren't interested in the answer, that's the real question isn't it?
 
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jodomcfrodo said:
Huh height is the only way that is truly 100% accurate due to the weight of the vehicle compressing the tires.
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Revolutions per mile is the same as the height so measuring the length for one revolution gives you the height, if you measure over 10 revolutions you get the height with 10 times less error.
 
pagrey said:
Revolutions per mile is the same as the height so measuring the length for one revolution gives you the height, if you measure over 10 revolutions you get the height with 10 times less error.
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Essentially a circumference measurement, right?
 
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