Rim Width Range
Because tires have flexible sidewalls, a single tire size will fit on a variety of rim widths. A tire's rim width range identifies the narrowest to the widest rim widths that the tire is designed to fit. The width of the rim will influence the width of the tire. A tire mounted on a narrow rim would be "narrower" than if the same size tire was mounted on a wide rim. NOTE: Because the overall diameter of a steel belted radial is determined by the steel belts, there is little, if any, change to the overall diameter of the tire due to differences in rim width.
The industry rule of thumb is that for every 1/2" change in rim width, the tire's section width will correspondingly change by approximately 2/10".
For example: a tire in the P205/60R15 size is measured on a 6.0" wide wheel and this size tire has an approved rim width range from 5.5" to 7.5" wide. The tire has a section width of 8.23" (209mm) when mounted on a 6.0" wide wheel. If that tire were mounted on all of the rims within its range, the tire's approximate section width would change as follows:
<table border="1" cellpadding="2" cellspacing="2" width="275"> <tbody><tr> <th align="left">Difference from
Measuring Rim</th> <th align="left">Rim
Width</th> <th align="left">Approximate Tire
section width</th> </tr> <tr> <td>0.5" narrower</td> <td align="center">5.5"</td> <td align="center">8.03"</td> </tr> <tr> <td>Measuring Rim</td> <td align="center">6.0"</td> <td align="center">8.23"</td> </tr> <tr> <td>0.5" wider</td> <td align="center">6.5"</td> <td align="center">8.43"</td> </tr> <tr> <td>1.0" wider</td> <td align="center">7.0"</td> <td align="center">8.63"</td> </tr> <tr> <td>1.5" wider</td> <td align="center">7.5"</td> <td align="center">8.83"</td> </tr> </tbody></table>
Because of the different wheel widths used in the above example, there is a 8/10" projected difference in tire section width when comparing a tire mounted on the narrowest rim to the widest rim within its range. This may affect fenderwell and frame clearances when selecting optional aftermarket wheel and tire packages
Additionally, some vehicle manufacturers and tire companies have permitted rim widths that are not within the tire's original approved rim width range. For example: BMW has combined 235/40R17 sized tires on 17x7.5" rims (which are 0.5" less than the narrowest 8.0" wide rim listed for the size) on certain M3 models; and Chevrolet has combined P255/50R16 sized tires on optional 16x9.5" rims (which are 0.5" wider than the 9.0" wide rim now listed for the size) on certain Corvette models. While these applications have received the approval of the vehicle and tire manufacturers, staying within the approved rim width range helps assure that the tire's internal stresses are within its design parameters.
Measuring Rim Width
The measuring rim width is the industry standardized rim width upon which the tire must be mounted in order to confirm it meets its dimensional targets. Because the width of the rim will influence the width of the tire, a standard rim width for every tire size is assigned and must be used. This standardized measuring rim width allows all of the tires produced around the world to meet the same dimensional standards and therefore, be equivalent with regards to their physical size. The measuring rim width is sometimes referred to as the tire's "design rim width".
The assigned measuring rim width changes with the tire size's section width and with the tire size's aspect ratio. As tire section width increases, the measuring rim width increases proportionately in 1/2" increments. Therefore, relatively narrow wheel widths are assigned for smaller tires while wider wheel widths are assigned for larger tires.
<table border="1" cellpadding="2" cellspacing="2" width="175"> <tbody><tr> <th align="center">Tire Size</th> <th align="center"> Measuring
Rim Width
</th> </tr> <tr> <td>P175/75R14</td> <td align="center">5.0"</td> </tr> <tr> <td>P195/75R15</td> <td align="center">5.5"</td> </tr> <tr> <td>P215/75R15</td> <td align="center">6.0"</td> </tr> <tr> <td>P235/75R15</td> <td align="center">6.5"</td> </tr> </tbody></table>
Additionally, relatively "narrow" measuring wheel widths are assigned for taller profile tires (75-series sizes) which graduate in 1/2" increments to the wider wheel widths assigned for lower profile tires (40-series sizes)
<table border="1" cellpadding="2" cellspacing="2" width="189"> <tbody><tr> <th align="center">Tire Size</th> <th align="center"> Measuring
Rim Width
</th> </tr> <tr> <td>P225/75R15</td> <td align="center">6.0"</td> </tr> <tr> <td>P225/70R15</td> <td align="center">6.5"</td> </tr> <tr> <td>P225/65R16</td> <td align="center">6.5"</td> </tr> <tr> <td>P225/60R15</td> <td align="center">6.5"</td> </tr> <tr> <td>P225/55R15</td> <td align="center">7.0"</td> </tr> <tr> <td>P225/50R15</td> <td align="center">7.0"</td> </tr> <tr> <td>P225/45R17</td> <td align="center">7.5"</td> </tr> <tr> <td>P225/40R18</td> <td align="center">8.0"</td> </tr> </tbody></table>
Section Width
A tire's section width (also called "cross section width") is the measurement of the tire's width from its inner sidewall to its outer sidewall (excluding any protective ribs, decorations or raised letters) at the widest point. This measurement is made without any load placed upon the tire and after the tire has been properly mounted on its industry assigned measuring rim and has been inflated and reset to its test pressure after 24 hours.
Because a tire's section width is influenced by the width of the rim upon which the tire is mounted, the correct industry assigned measuring rim width for the tire size being measured must be used.
The width of a tire mounted on a narrow rim would be "narrower" than if the same tire was mounted on a wide rim. NOTE: because the overall diameter of a steel belted radial is determined by the steel belts, there is little, if any, change to the overall diameter of the tire due to differences in rim width.
The industry rule of thumb is that for every 1/2" change in rim width, the tire's section width will correspondingly change by approximately 2/10".
For example: a tire in the P205/60R15 size is measured on a 6.0" wide wheel and this size tire has an approved rim width range from 5.5" to 7.5" wide. The tire has a section width of 8.23" (209mm) when mounted on a 6.0" wide wheel. If that tire were mounted on all of the rims within its approved range, the tire's approximate section width would change as follows:
<table border="1" cellpadding="2" cellspacing="2" width="268"> <tbody><tr> <th align="center">Difference from
Measuring Rim</th> <th align="center">Rim
Width</th> <th align="center">Approximate Tire
section width</th> </tr> <tr> <td align="center">0.5" narrower</td> <td align="center">5.5"</td> <td align="center">8.03"</td> </tr> <tr> <td align="center">Measuring Rim</td> <td align="center">6.0"</td> <td align="center">8.23"</td> </tr> <tr> <td align="center">0.5" wider</td> <td align="center">6.5"</td> <td align="center">8.43"</td> </tr> <tr> <td align="center">1.0" wider</td> <td align="center">7.0"</td> <td align="center">8.63"</td> </tr> <tr> <td align="center">1.5" wider</td> <td align="center">7.5"</td> <td align="center">8.93"</td> </tr> </tbody></table>
Because of the different wheel widths used in the above example, there is a 9/10" projected difference in tire section width when comparing a tire mounted on the narrowest rim to the widest rim within its range. This may affect fenderwell and frame clearances when selecting optional aftermarket wheel and tire packages
Tread Width
The tread width is the distance between the outer edge and the inner edge of the tread of a new tire. However today's radial tires often feature tread designs that incorporate rounded shoulders and there is no industry standard pertaining to "how much" of the rounded shoulders should be included in the tread width measurement. Because of this, it is difficult to accurately compare the tread width differences of one tire brand to another. Tread width measurements are best used when comparing the various tire sizes or lines manufactured by a single tire manufacturer. Several tire manufacturers have chosen not to publish tread width dimensions.
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