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 Tire size an what the numbers mean...

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PostSubject: Tire size an what the numbers mean...   Tue Jan 10, 2012 12:41 pm

The Us Tire and rim association Inc., has established standard designation and interchangability standards for tires, rims, and related components. Example: P235/70/R15
The "P" Stands for Passenger car tire.
The "235" is the section width.(235 Millimeters. Measured from the rim mounting surface to the edge of tire).
The "70" is the aspect ratio.
The "R" Means Radial tire.
The 15 Means for a 15 inch wheel.
First, the tires Sectional Width (Also known as it's cross-sectional width), is the distance from the widest point of the sidewall on the inside to the widest point on the outside. "Bulge to bulge" so to speak, with no load on tire.
Always expressed in millimeters. To convert millimeters to inches multiply by 0.03937. (Ex: 235 X 0.03937=9.25195 inches).
Of course, in the real world a tires total width depends on what size rim (Width) it is mounted on, as well as inflation pressure.
So, a tire that is 235 Millimeters section width [Or 9.25195 inches) will only be that wide when mounted on the industry's standard rim width and tire pressure (35 PSI) for that size tire.
If the tire is mounted on a narrower or wider wheel than specified, the section width becomes correspodingly narrower or wider. The rule of thumb is that for every 0.5-inch change in rim width changes the sectional width approx. 0.2....
A tires sidewall aspect ratio or profile (70 above) indicates the tires section height as a percentage of the section width.
Section height is 70 percent of the section width The higher the number, the taller the tire. Measured for the rim to the edge of tread.
One would think that two tires with the same section width should have the same width regardless of aspect ratio, but this isn't nessesarily the case.
As stated, only if the tires are mounted on the standard specified width rim for that size tire.

On the other hand, a 295/5015 tire would be 50% as wide as it is tall. (Very wide tire, with a sidewall of almost 150 millimeters).
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PostSubject: Re: Tire size an what the numbers mean...   Tue Jan 10, 2012 3:27 pm

Thanks for posting this Chaney, Lot of good info, Smile


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