Tire Tread Width vs. P(num)?

jerry_njAugust 2, 2005

I know that the first number of a metric radial tire is the width of the tire at the sidewall. For example a P195/65R is 195 millimeters wide at the sidewall. Also the tire sidewall height is 65% of 195 millimeters. This tire is a "low profile" sporty tire, and I have a set on one vehicle. The tires look very wide (and are a bit noisy).

I also have P205/75R on another vehicle. Now I would assume the tread width of the P205 tire would be wider than the tread width of the P195 tire, it isn't. It must be that the "low profile" fire has straighter sidewalls, and the tread has a more squared off transition to the sidewall than does the taller 75% tire.

Could this be correct or is there something wrong with my measuring, and my eye for that matter. It is the fact the 195 tire looked wider that caused me to measure.

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I believe the width is measured with the tire off the vehicle on the inside part where it fits up against the rim. That would make sense, anyway, because it's there that the width is most critical in terms of what tires will fit on what rims.

For what it's worth, I've noticed similar oddities in tire measurements. The tires on my S10 are supposed to be 31" in diameter, but on the truck they measure about 29.5". Maybe sitting on a shelf at the store they are taller than when they have 2 tons resting on them.

    Bookmark   August 3, 2005 at 4:29AM
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Yes, the measurements are off the vehicle. This would account for the difference in diameter you found.

My reference says: "..measured from sidewall to sidewall, when it is properly mounted on a wheel and inflated but not supporting a car." I take this to mean at the wide part of the sidewall, not at the rim. I also assume I can replace a P205/15 with a P225/15 when the tires wear out, using the same rims. Not sure about this last point.

    Bookmark   August 3, 2005 at 12:12PM
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Attached is a link showing the tire dimensions and thier definitions. You're question pertains to the "section width" value as opposed to the "overall section width"

If you follow the link to the "tire sizes" in the upper right hand corner it will show this.

cowboy the dimension where you refer to (at the bead) is not a fixed dimension due to the flexibility of the side wall. FWIW, It is possible to mount the same size tires on different width rims. I suspect if it alters the section width from the specified value it would probably void any warranty as an improper application.

Hope this helps.

Here is a link that might be useful: tire dimensions

    Bookmark   August 3, 2005 at 1:41PM
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Thanks, and the figure does recognize my concern: tread width, but I could not find any tire size that also gave a tread width. It may just be too late and I can't see .. better get some sleep, this question will not interfer with my sleeping.

    Bookmark   August 3, 2005 at 11:31PM
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