Tire size

[nutensteve]

I'm sure this question has been answered before but I'll ask it again. I have to replace the Good Year 275/80R/22.5's on the front of my 06 diplomat. I'm leaning towards the Michelin Tire. The tire dealer has recommended that I up the size to 295/80R's. I was told that the tire only has an increase of 1/2" in diameter. Has anyone changed to this size on a Diplomat? I'm a little concerned about clearance in the wheel well.

Thanks,
Steve

[Theberrys]

With this data you can caculate the sizes of both tires.


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.

Example size: 225/45R17
The first three numbers in a typical size (225/45R17) 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:

225mm / 25.4 = 8.86"

The second pair of numbers (225/45R17) 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:

225mm x 0.45 = 101.3mm
8.86" x 0.45 = 3.99"

The last number (225/45R17) 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:

17" x 25.4 = 431.8mm

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.

101.3mm + 101.3mm + 431.8mm = 634.4mm
3.99" + 3.99" + 17" = 24.98"

WARNING: These calculations (and those provided by all online tire dimension calculators) only reflect nominal tire dimensions, which are often subtly or substantially different than the tire's actual physical size and/or the dimensions provided by the tire manufactures.
Drivers working with close fitment tolerances or trying to maintain the original tire diameter of staggered size fitments (similarities or differences) should use the tire manufacturer's specifications for the exact tire(s) they are considering.
Actual Tire Section Width is Dependent on Wheel Width

All tire sizes are assigned specific rim widths upon which they are measured (measuring rim), as well as can often be mounted on slightly narrower or wider wheels (rim width range). Therefore, it is important to note that actual tire section width will depend on the wheel width 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 then measuring wheel and increased when mounted on wider wheels.


Dick

[OneRVer]

Thanks for that info Dick. I have been looking for this and could not find it. Where did you find it?

[Theberrys]

I found it here
Tire Tech Information - Calculating Approximate Tire Dimensions

Dick

[nutensteve]

Thanks for the detailed size calculator Dick. I'll try to figure the diameter and see if it will cause any clearance problems.

[Mr_D]

Quote:

Originally Posted by nutensteve Thanks for the detailed size calculator Dick. I'll try to figure the diameter and see if it will cause any clearance problems.

Personally I'd go to the tire manufacturers sites and get the real info and compare them. The site provided warns that what you get is only an approximation.

Quote:

WARNING: These calculations (and those provided by all online tire dimension calculators) only reflect nominal tire dimensions, which are often subtly or substantially different than the tire's actual physical size and/or the dimensions provided by the tire manufactures. Drivers working with close fitment tolerances or trying to maintain the original tire diameter of staggered size fitments (similarities or differences) should use the tire manufacturer's specifications for the exact tire(s) they are considering. Actual Tire Section Width is Dependent on Wheel Width

Even changing the wheel width has an impact.

[rpasetto]

Also, make sure the width of your existing rims can handle the upgraded tire size.

[Robert T]

If you change tire size by changing over all tire diameter there will be change in speedometer accuracy but this can be corrected by changing the dip switches on the back of the speedo head.

If you call Monaco they will send you by E-mail a calulation sheet on how to set these dip switches. You will need axle ratio, tires size to be used and trans final drive ratio. The flow chart will tell how to set each dip switch.