Am I dreaming of are these 33s really 32s...

[Onward4x4]

So I have installed the Toyo Open Country R/t 33x12.5-18s inflated at 36 on Rhonda the Jeep.
I measured from top to bottom and they are 32s... Am I dreaming or doing something wrong? Is this right?

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[chacomaya]

So I have installed the Toyo Open Country R/t 33x12.5-18s inflated at 36 on Rhonda the Jeep.
I measured from top to bottom and they are 32s... Am I dreaming or doing something wrong? Is this right?

I believe these numbers are "nominal", not exact measurements. I know from motorcycle tires that one manufacturer's 180mm width tire can actually measure out as wider than another manufacturer's 190mm tire. It's not the norm, but it does happen.

 

[MKJL]

Your spare without a load is probably 33” but that’s normal once they have weight on them to compress some.

 

[NFRs2000NYC]

The measurement on the tire is uninflated and uninstalled. With the weight of the vehicle, the tire will measure smaller, and different manufacturers will be off by more/less. BFGOODRICH for instance run small, so their 37" tire will measure out to 35.5 or so, while other makes will run nearly 36".

 

[chacomaya]

Your spare without a load is probably 33” but that’s normal once they have weight on them to compress some.

That is a good point. I wonder if you did the measurement along the horizontal if you might get closer to 33".

 

[NFRs2000NYC]

To get a good measurement, measure from the ground to the center of the centercap, and multiply by 2.

 

[Onward4x4]

So if the diameter is 32, I should make sure the speedometer is set for 32 as well--at least my logic dictates... Yes?

 

[Onward4x4]

To get a good measurement, measure from the ground to the center of the centercap, and multiply by 2.

That is a good idea. I will try that method as well just to make sure. Thanks!

 

[Spank]

So if the diameter is 32, I should make sure the speedometer is set for 32 as well--at least my logic dictates... Yes?

Put the true measurement of the tire under load. Don't just punch in the tire size.

 

[NFRs2000NYC]

So if the diameter is 32, I should make sure the speedometer is set for 32 as well--at least my logic dictates... Yes?

On the JK, you needed to do the measured size described in my previous post. With the JL, it seems people input 37 for 37 and it comes out accurate. Best thing to do is to get a GPS speedometer app for your iphone or whatever, get it up until the phone read 65, and verify on the speedometer on the Jeep, and adjust accordingly.

 

[Onward4x4]

On the JK, you needed to do the measured size described in my previous post. With the JL, it seems people input 37 for 37 and it comes out accurate. Best thing to do is to get a GPS speedometer app for your iphone or whatever, get it up until the phone read 65, and verify on the speedometer on the Jeep, and adjust accordingly.

I think I will do that. I punched in 32 and the vehicle is pretty spunky just like on stock tires--and it gets into 8th speed when it feels it should. I had punched in 33 originally and I felt a bit of a difference and sluggishness. I'll take your advice and verify it using an App. Thanks for the tip!

 

[WXman]

We need a sticky at the top of the page so this doesn't keep coming up every week.

Manufacturer listings are done on a specific width wheel, at a specific PSI, including the bulge of the sidewalls. The numbers you see listed in their charts will be done this way.

You and I measure on our own wheels, at our own PSI, usually with the weight of the vehicle on them, and most importantly, in a straight line without the bulge of the sidewalls.

That is why we get lower tire heights than what we see listed in charts. It's not that the tire data is wrong, it's just a difference in how we measure tires.

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