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Ford Truck Club Forum > GARAGE TALK > Garage Talk: Wheels & Tires



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Old January 8th, 2012, 01:18 AM
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HawaiianF150 HawaiianF150 is offline
 

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Default How To Read Your Tire's Numbers

How To Read Your Tire's Numbers

You should always use the correct tire for the vehicle. The correct tire size, air pressure and load ratings can be located on a sticker usually located on the driver side door/door jam or in the vehicles manual.
Most modern passenger car and light truck tires feature size designations that indicate the tire's dimensions in a combination of metric, mathematical and English systems. While this unusual combination of millimeters, percentages and inches is a byproduct of the evolution of global tire specifications, it also provides the ability to calculate/estimate basic tire dimensions.

See The Atttached Picture Below:

Example size: 215/70R14
The first three numbers in a typical size (215/70R14) are the tire's indicated section width in millimeters, measured from sidewall to sidewall.

If you are familiar with measurements in inches, the section width in millimeters can be converted into inches by dividing it by 25.4.
For example: 215mm / 25.4 = 8.5"

The second pair of numbers (215/70R14) is the tire's aspect ratio or profile.
This is a ratio of sidewall height to section width.
The section height's measurement can be calculated by multiplying the section width by the aspect ratio. The answer will be the height of one sidewall.

For example:
215mm x 0.45 = 96.75
8.85" x 0.45 = 3.825" (ratio of sidewall height to section width which is the height of one sidewall)

The last number (215/70R14) is the diameter of the wheel in inches.

If you are familiar with measurements in the metric system, the wheel diameter can be converted into millimeters by multiplying it by 25.4. For example: 14" x 25.4 = 355.6mm

To calculate the overall diameter of a tire, the sidewall height must be multiplied by 2 (remembering that the tire diameter is made up of 2 sidewalls, the one above the wheel and the one below the wheel touching the ground) and add the diameter of the wheel.

For example: 96.75(top sidewall height in mm above the wheel/rim) plus 96.75(bottom sidewall height in mm below the wheel/rim) equals 193.5

When we add that into the measurement of your wheel / rim size, we get the total diameter of the tire and wheel/rim which is 355.6mm (your 14in wheel size) to your sidewall measurements of 193.5 . All of those measurements equal 549.1mm or [/B] as measured in inches,

The overall tire and whell diameter is 3.825" + 3.825" + 14" = 21.65 inches.

Diameter is the measurement from top to bottom across the centerline.
Radius is the measurement from an outside starting point going all the way around the outside back to the original starting point.

When you are not sure or in doubt of what tire to use - you should always use the vehicle manufacturer's tires pecifications, usually located on the drivers door jam.

When working with close tolerances such as fenderwells, frame and steering components or even trying staggered sized tires consult your local tire shop or mechanic before you notice your tire passing you you drive down the road. Hospital bills aren't cheap.

Actual Tire Section Width is Dependent on Wheel Width
All tire are manufactured for specific rim widths. Some can be mounted on slightly narrower or wider wheels (rim width range) but remember the hospital bill and the freaky feeling of watching your tire pass you. Besides it just sux when your tire leaks air each time you hit a bump.

It's important to note that actual usable tire section width will depend on the rims wheel width that the tire is mounted on. The rule of thumb is that tire section width changes by 0.2" for every 0.5" change in rim width, being reduced if mounted on narrower wheels and increased when mounted on wider wheels.
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Last edited by HawaiianF150; March 7th, 2012 at 01:12 AM.


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