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Understanding Off-Road Tire Size Measurements

Off-road tire size designations are different than normal tires. Here, we explain these measurements and share some helpful pointers that are essential during the selection process.
Chevy C/K Blazer On Deep Thread Off-Road Tires

When you want higher levels of grip for stomping over mud, rocks, or snow, off-road tires are typically designed with deeper tread, larger sizes, and higher sidewall ratios than standard tires. This helps you in rough spots - whether you're keeping your 4x4 at stock ride height, or raising it up with either a body or suspension lift kit.

When shopping for tires designed specifically for off-road use, you'll come across size designations that are different from traditional measurements. "35 x 12.5-17" is one such example. Here, we'll help you understand this type of size, and we'll touch on a few helpful pointers to make your selection process easier.

Off-road Tires Are Referred To Primarily By Outer Diameter (In Inches)

Off-Road Land Rover

The first thing to understand is that, when it comes to tire specs, off-road tires are often described primarily by a measurement of their outer diameter, in inches. There are, of course, other important tire dimensions to consider – but total outer diameter serves as a key tire size comparison reference for figuring out if you’ll exceed the amount of clearance inside your vehicle’s wheel wells.

That clearance inside the wheel wells is also measured in inches, and it’s important not to exceed that capacity if you want to avoid the headaches of tires rubbing against fenders, wheel well liners, or control arms. Tire diameter can easily be figured out looking at any standard tire size measurement.

How To Calculate The Outer Diameter Of Your Tires, In Inches

Tire Size Conversion

BridgeStone Dueler Off-Road Tire
For reference, we'll use the 245/75-16 tire shown in this photo.

Using a standard tire size of 245/75-16 as an example, it's important to note those first two numbers use millimeters as a measurement reference, not inches. The "245" refers to the fact that this tire is 245 millimeters wide. The "75" number refers to the fact that the sidewall height is 75 percent of the 245 mm tire width. So in this case, 245mm x 75% = 183.75 millimeters of sidewall height.

To convert that 183.75mm number into inches, multiply it by 2, then divide by 25.4. The resultant number of 14.47 is this tire’s sidewall height in inches.

Add in Wheel Size

Next, take the last number “16” (diameter of the wheel) in the 245/75-16 original measurement and add it to the 14.47” tire sidewall height. Since 16 + 14.47 = 30.47, the total diameter of this tire in inches is 30.47.

Know How Much Tire Will Fit On Your Vehicle

Modded Off-Road Vehicle On Large Tires

If you’re keeping your truck’s ride height stock, the vehicle manufacturer can provide reliable info when it comes to tire clearance – via the owner’s manual, or by a quick call to their customer service department.

If you are installing a lift kit, you’ll gain some inches of clearance. Because of suspension design and layout, the exact amount of gain will vary from vehicle to vehicle. Internet forum pages are a great place to gauge the experience and advice of other vehicle owners on the subject of how much lift is necessary for certain tire sizes. Most lift kit manufacturers provide suggested tire sizes for each of their kits as well. For perspective, our related article How Much Lift Is Needed For Larger Tires On My 2007-Up Jeep Wrangler? lets you visually compare tire sizes on Wranglers. Pictures in the article show various tire sizes fitted to Wranglers that have been lifted to a range of heights.

Understanding Off-road Tire Size Designations

Across the internet, you’ll see lots of references to off-road tire diameters and widths in fractions of an inch (“31.8 x10.8–16” for example). It’s important to understand people are doing this because they've converted the measurements of metric tires, as we did earlier in this article. However, new tires larger than 445mm in width are available in off-road sizes on our website - and only in whole number diameters. So on our selection pages, you’ll see sizes such as 27, 28, 32, etc. You won’t see anything like 29.3, or 31.8 available for sale.

Off-Road Tire Size Designations
After entering year, make, model, and vehicle lift height, our website may present you with both types of tire size choices.

The First Number in Off-Road Tire Sizes

Total Diameter Of Off-Road Tire

The first number displayed on off-road tire measurements is the total diameter of the tire, in whole inches.

The Second Number in Off-Road Tire Sizes

Width Of Off-Road Tire

The second number of the off-road size measurement is the width of the tire, expressed in inches. If we were converting the width of our tire in the picture above to an off-road dimensional reading, we would take the width (245 millimeters) and divide it by an adjustment factor of 25.4 to equal 9.64 inches.

If you are purchasing a new off-road tire, you’ll find widths are available only in whole and half-inch dimensions. For example, 9” or 9.5”.

The Third Number in Off-Road Tire Sizes

Inner Diameter Of Off-Road Tire In Inches

The third number in an off-road tire measurement is unchanged from the standard one. It signifies the diameter of the wheel in inches. Therefore, an off-road tire with a third number of “16” means it’s for a 16-inch rim.

Metric And Off-road Tire Sizes Sometimes Overlap

When you’re shopping tires for your 4x4, it’s important to consider that tires with standard metric measurements can sometimes have size overlaps with those built to off-road inch specifications. What we mean is, “equivalent” diameters of metric and off-road tires are not actually equal in most cases. For example, our tire size of 245/75-16 with a diameter of 30.47” would place it smack between equivalent off-road tire sizes of 30” and 31”.

Ultimate VW 4x4er

If you were not changing your vehicle's ride height and clearance in the wheel well was already tight, we'd recommend playing it safe and choosing the smaller 30" size. If you've raised your vehicle or have plenty of additional clearance, then it's ok to choose larger sizes.

To ensure you see all sizes available that may be appropriate, shopping BOTH off-road and standard metric sizes before making a final purchase is advisable.

We trust that this brief overview of off-road tire sizing has clarified how you might compare standard metric and "inch-sized" tires. We prefer that you're armed with this knowledge to make the shopping experience rewarding and fun!

Tire Size Guide

If you’ve used our tire size converter formulas in section 2 of this article, you now have a reference point for what your factory ones would measure on the off-road tire scale. Using that number as a starting point will make tire comparison easier, because it’s easy to envision how a larger or smaller off-road tire will look compared to what you already have.

If you prefer to think in terms of traditional tire measurements when looking for tires for your 4x4, the tire size guide below lets you see both standard and off-road sized tires we offer for each wheel diameter. For example, the 15” Tires drop down box shows typical tire sizes for 15-inch wheels – and so on.

Tire Size Chart

17.5" TIRES
19.5" TIRES
22.5" TIRES
24.5" TIRES

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