Tire Size Discrepancy

[DelawareGuy84]

Hey Everyone,

I’m a bit confused, and hope that one of you fine folks can help me out:

I have a JT Sport S and am about to purchase a new set of JL Rubi wheels and tires.

According to tire size calculator, the JT Sport S tire size is 31.5” but when I measure them while on my JT, they only come out to about 31”. Is this a common discrepancy? I am worried that when I calibrate the new JL Rubi wheels (32.7” according to TSC) that I’m going to be off.

Thanks in advance for any help you can provide!

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[SandSurfer]

It will not be a problem.

 

[Incommando]

The factory height is measured without the weight if a vehicle on them if IIRC

 

[SandSurfer]

A half inch bigger or smaller is NOT going to make any difference

 

[DelawareGuy84]

Okay thank you all, I very much appreciate it!

So when I go to calibrate the speedometer, I should put in the TSC size of the JL Rubi wheel (32.7”)?

 

[jwilson2899]

Okay thank you all, I very much appreciate it!

So when I go to calibrate the speedometer, I should put in the TSC size of the JL Rubi wheel (32.7”)?

If it helps out, when I went to reprogram my Rubi for 35’s, the Tazer showed the setting from the factory to be 32.75 for the stock Rubi tires. Based on that I think 32.7 should work just fine for you.

 

[Factoid]

Size matters...except in tires. Manufacturers use a common size, but can vary quite a bit from that standard size. Stated tire size is directionally correct, but must be verified if the dimension is critical.

 

[WXman]

Manufacturers measure tires on a specific width wheel, at a specific air pressure, without a load on them, and WITH the bulge of the sidewalls.

You and I measure tires on our own wheels, at our own air pressure, with a load on them, in a straight line.

Obviously we're going to get two totally different numbers.

As far as setting your Tazer, here's what you should do: Lay a level on the top of your tire, dead center. Run a tape measure from the ground to the bottom edge of the level. That'll tell you exactly how much diameter you need to input when you program your speedo. A Rubicon tire is generally going to fall in the 32.2" range. A Sport tire is generally 31".

 

[DelawareGuy84]

Manufacturers measure tires on a specific width wheel, at a specific air pressure, without a load on them, and WITH the bulge of the sidewalls.

You and I measure tires on our own wheels, at our own air pressure, with a load on them, in a straight line.

Obviously we're going to get two totally different numbers.

As far as setting your Tazer, here's what you should do: Lay a level on the top of your tire, dead center. Run a tape measure from the ground to the bottom edge of the level. That'll tell you exactly how much diameter you need to input when you program your speedo. A Rubicon tire is generally going to fall in the 32.2" range. A Sport tire is generally 31".

So when you put the size in the Tazer, it’s the size YOU measure, not the sidewalk size?

 

[Incommando]

So when you put the size in the Tazer, it’s the size YOU measure, not the sidewalk size?

Right

 

[WXman]

So when you put the size in the Tazer, it’s the size YOU measure, not the sidewalk size?

Yes, the actual diameter while it's on the truck. I've done that with a JL and JT, and with 4 different sets of tires of various sizes, and when I check the speedo vs. GPS it's spot-on.

 

[j.o.y.ride]

Hey Everyone,

I’m a bit confused, and hope that one of you fine folks can help me out:

I have a JT Sport S and am about to purchase a new set of JL Rubi wheels and tires.

According to tire size calculator, the JT Sport S tire size is 31.5” but when I measure them while on my JT, they only come out to about 31”. Is this a common discrepancy? I am worried that when I calibrate the new JL Rubi wheels (32.7” according to TSC) that I’m going to be off.

Thanks in advance for any help you can provide!

You can lose 1/2" from a tire like a M/T just wearing down. My Trail Grapplers have a tread depth of 0.6 new. When that wears to 0.35 I will have lost .50 on the diameter.

It doesn't matter.

 

[brianinca]

Exactly, when I swapped out my BFG KM2's from my YJ for the take-offs from my JTR, the 33x10.5 BFG's had "shrunk" to 31.8 from 32.5, which is where the Falken AT's were. Good tread life for an MT though, lots of 1,000 mile trips on them in five years.

You can lose 1/2" from a tire like a M/T just wearing down. My Trail Grapplers have a tread depth of 0.6 new. When that wears to 0.35 I will have lost .50 on the diameter.

It doesn't matter.

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