Understanding The Measurement of Off-Road Tires!

The first modification that most off-road truck owners make before adding things like suspension lifts or custom bumpers such as steel truck bumpers is to switch out their stock wheels and tires for bigger ones. What does “bigger” mean?

Before you can decide which tires you need, you have to know the different ways off-road tires are measured. Understand these different dimensions so you choose the best tires for your off-roading truck or Jeep.

How Should You Decide on Off-Road Tire Size? 

When upgrading your off-road tires, choosing the right size is essential to get the performance you need while making sure they still fit under the truck  you take off-roading. To do this, you need to know a few important measurements so you can figure out which size you want and how big you can actually go.

All tires are typically measured in inches according to their outer diameter. This measurement is critical for determining how much tire you can fit under your wheel wells. Width that usually increases proportionally with the diameter must also be considered when purchasing off-road wheels and tires.

These two dimensions account for the two measurements used to purchase a set of tires. Based on these measurements, off-road tires are then offered in a selection of diameter and width designations.

Calculating Tire Outer Diameter

Before you buy a new set of tires for your Jeep or truck, it's important to know the size that's on it right now. Based on this size rating, you can gauge how much space is available for larger tires and what choices you have.

Tire size that is considered for your off-road truck is usually written in a formula that includes tire width in millimeters, the percentage of tire width in millimeters as the sidewall height, then the wheel diameter in inches but not the overall outer tire diameter.

For example, a standard tire size would look like this: 245/75-16. Translated, this tire has a width of 245 mm, a sidewall height of 75% of the tire width which calculates to 183.75 mm, and a diameter in inches of 16.

The way you calculate the outer diameter of the tire is by converting sidewall height to inches and then adding in the wheel diameter:

  • 183.75 mm multiplied by 2, then divided by 25.4 = 14.75 inches of sidewall dimension.
  • Add 14.75 inches to 16 inches to come up with a tire diameter of 30.47 inches.

Off-Road Tire Dimensions and Size Designations

Figuring out the outer tire diameter is only half the concern when choosing tires for your off-roader. Instead of appearing as the standard tire dimensions of conventional tires, off-road varieties are listed by designation according to diameter size, width, and wheel diameter so you can immediately know which ones will fit your off-roader and which won’t.

A common example of an off-road tire designation looks like this: 35x12.5/18. In this group of numbers, the 35 is outer tire diameter in inches, the 12/5 is the tire width in inches, and the 18 is the diameter in inches of the wheel that must be used for the tire. If you find numbers documented as fractions of an inch, those are metric tires converted to imperial measurements.

Using the above calculations and the explanation on reading off-road tire designations, you can easily refer to your vehicle’s user manual to determine wheel well size and how large you can go when purchasing bigger tires. Keep your measurements within the allotted space to avoid clearance problems with your wheel wells, fender, and steel truck bumpers. Clearance issues can also impede articulation and affect the suspension.

If you have your heart set on tires that are bigger than what's recommended by your truck manufacturer, plan on adding a body or suspension lift kit as well. A lift will give you more space for bigger tires, greater articulation, and the other benefits you’re looking for by adding those monsters in the first place!

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