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Tire Sizes

The first question is, what size tire can I put on my vehicle? First we must look at what size will fit and how sizes are determined. In this article I will use a 225/75/16 tire. The average size was computed averaging the sizes posted on the different manufactures web sites for a M/T tire. The rim size came out of the owners manual.

Here are a few terms that need to be explained. This is based on tires with the correct inflation specified for the tire and load of the vehicle.

Aspect Ratio:
The “75”is known as the Aspect Ratio. It is calculated by dividing the section height by the section width and multiplying by 100. (In this example, the sidewall will be 75% of 225)

Overall Diameter: diameter of the tire from tread surface to tread surface while inflated but unloaded This is the number off roaders are most interested in. As in a 33inch tire.

Static Loaded Radius:
distance from the center of the axle to the ground under the specified load with recommended tire inflation.

Rim Diameter: diameter of the rim from bead seat to bead seat.

Overall Section Width:
distance between the outer sidewalls of an inflated tire.

Rim Width: distance between the inside of the rim flanges. The rim width is important to consider when looking for larger tires.

Section Height: distance from the bead seat to the outer tread surface of the inflated tire Section height effects the way your vehicle responds to input from the steering wheel. The taller the section height, the less responsive a tire generally is. This decreases as you “air down” for trails.

Section Width: distance between the outer sidewalls of an inflated tire, less any ornamentation, side lugs or curb ribs. The “225” is the width of a tire and is measured in millimeters when mounted on the correct rim and inflated to the recommended tire pressure. Section width increases as you “air down” for the trail. Be mind full of springs and steering gear that could rub with the inside of the tire.

Tread Width: the width of the tread surface, designed for contact with the road. Often referred to as footprint. Foot print also includes the length of the tread area on the ground and increases as you “air down” In theory the larger the foot print, the more traction.

Now all this said, here is the formula to figure out over all diameter of a tire. I will use 225/75/16 tire as an example. The width is 225, aspect ratio is 75 and the wheel size is 16.

1.Convert the width in millimeters to inches. This is done by dividing 225 by 25.4

225/25.4=8.86

2. Determine the section height by multiplying the aspect ratio. In this case 75

8.86X.75=6.645

3. Now all you have to do is add the two section heights to the rim size.

6.645+6.645+16=29.29

Your tire is 29.29 inches tall when inflated to the recommended pressure and your vehicle is not loaded (dry weight).

Now the answer to the question. How much ground clearance do I gain? Take the one section height for the current tire/wheel combination and subtract it from the new tire/wheel combination. Remember. You only gain one tire section height plus the difference in the rims diameter from the axle to the rims edge. Going from 15 inch to 16 inch in reality is ½ inch. The other half is above the axle and will not effect ground clearance. It does effect fender clearance.

Chart

Disclaimer:
These are not recommendations or suggestions, but only the information as it has been harvested from various sources including other web sites. They are presented for the informational use of the reader in making their own decisions. There are too many variables in tire use and applications to recommend a size for a particular use. Also there are too many brands, and they are like all opinions, very personal.

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