To venture off the beaten path and take on something a little more exciting than asphalt, off-road tires make a world of difference. Made with thicker tread, stronger sidewalls, and more aggressive tread patterns to evacuate mud, snow, and water, these tires are designed to hit dirt roads, crawl rocks, and take on everything in between.
While shopping for off-road tires, the sizes look different than the standard size convention of regular car and truck tires, which can be confusing at first. On a normal passenger car, the tire size might be something like 225/60R17. Stepping into the world of off-road tires, the sizes look like 33x10.50-15, which is quite a bit different. So, what gives?
As the go-to tire guy among my friends after years of selling and installing them, I’ve had to explain the different sizing conventions quite a few times, and that’s the goal of this article. Knowing how tire sizes work makes it easier to swap from regular to off-road tires as needed to plan the perfect gravel-road adventure.
Why Off-Road Tire Diameter And Height Is So Important
Arguably the most important thing to know about off-road tires is that they’re commonly referred to by their outside diameter in inches. Also known as the height, this is usually how people talk about their tires. In a group of off-roaders, someone might say they have “33s” or “35s” on their truck or Jeep, which just means that the diameter (and height) of the tires is 33- or 35-inches.
Knowing a tire’s height is critical to ensure there is enough clearance within the wheel well, which is the circular opening in the side of a vehicle where the wheel and tire are situated. Staying within the wheel well spec, and not exceeding it, will help to avoid the headache of tires rubbing against fenders, wheel well liners, or control arms.
Tire diameter is also key because it can affect the accuracy of the speedometer. Installing bigger ties on a vehicle causes the speedometer to read lower than the actual speed since the tires travel more distance with each revolution. Recalibrating the speedometer using a scan tool or programmer can solve that issue, but before that happens, knowing the approximate speedometer error can prevent an unintentional speeding ticket.
Adding a lift kit or cutting the fenders is an easy way to increase clearance within the wheel well, but not everyone wants to start making changes just to install new tires. If that’s the case, sticking to an off-road tire size that’s close to the factory-installed size is an easy way to be sure it’ll fit.
To find this size, pop open the driver’s door and look for a tire information placard on the door jamb. Once the standard size from the placard is converted to the off-road tire size convention, it’s easy to find a set that fits without any mods, and that starts with the tire’s diameter.
How to Calculate the Outer Diameter of TiresThe first thing to know about regular tire sizes is that they use a strange combination of metric and standard measurements, combining millimeters, percentages, and inches. For the sake of this example, consider a 245/75R16, a common tire size found on smaller trucks and Jeeps.
To start, the 245 in the size means that the tire is 245 millimeters wide. Next, 75 refers to the aspect ratio and means that the sidewall is 75-percent of the tire width. So in this case, 245 millimeters x 75-percent = 183.75 millimeters of sidewall height, which then must be doubled since the sidewall appears on each side of the wheel. After all this math, there’s a total of 367.5 millimeters in sidewall height.
Next, that number can be converted to inches by dividing by 25.4, the number of millimeters in one inch. This means that the sidewall accounts for 14.47 inches of the total height in a 245/75R16 tire. That’s all the hard stuff out of the way, phew!
From there, adding in the wheel size is easy since it’s already in inches, and it’s simply the last number in the tire size, typically following the R. Looking at the example tire, it’s designed to fit a 16-inch wheel, and adding this to the sidewall height leads to a total tire diameter of 30.47 inches.
Understanding Off-Road Tire Size Designations
With the diameter calculated, finding some off-road tires that fit without making any modifications is easy. Spoiler alert: All of the numbers in off-road tire sizes are already in inches, making them easy to understand without having to do any millimeter-to-inch conversions or any other math.
Take a 30x9.50-16 as an example. The first number in an off-road tire size is the tire’s diameter or height. There are no metric measurements or percentages to deal with here. Using the example tire, the 30 at the front of the size means it’s a 30-inch tire, so the diameter (or height) is simply 30 inches.
The second number is the width of the tire, and it is also already expressed in inches. With the 30x9.50-16 example, the 9.50 means the tire is 9.50 inches wide, measured shoulder-to-shoulder across the tread. A heck of a lot easier than dividing the millimeters by 25.4, right?
The third and final number in an off-road tire size is the same as the standard size convention — wheel diameter. With the example tire, this would just be the 16 at the end, indicating, again, that it’s designed to fit on a 16-inch wheel.
Metric And Off-Road Tire Sizes Are Close But Not The Same
These example sizes might have seemed random at first, but they were used because the measurements are pretty close across the board once all the conversions are done. Expressed as an off-road size, the 245/75R16 tire would be 30.47x9.64-16, not too far from 30x9.50-16. This shows how standard and off-road tires can be close in size, but they’re rarely actually equal.
For better or worse, off-road tire sizes use nice round numbers, making them easier to deal with, while simultaneously making them harder to match regular sizes. A diameter of 30.47” puts the 245/75R16 smack between off-road tire sizes of 30- and 31-inch, while the width of 9.64 inches falls somewhere between 9.50 and 10.00 inches wide. This happens across the board with pretty much all regular tire sizes once converted, and it can present problems when making the swap.
Going to off-road tires with the examples in mind, playing it safe and choosing the smaller 30x9.50-16 instead of upping the size to something like a 31x10.50-16 will ensure that it fits with no clearance issues. That said, if there is plenty of clearance in the wheel well or a lift kit is being installed, squeezing in a 31-inch tire might not be a problem and it’ll make the vehicle even more capable in the mud and rocks.
Get The Most From An Off-Road Vehicle With Some Bigger Tires
Most vehicles come from the factory with a relatively conservative tire size, and there’s often a bit of room to play with in the wheel wells to go a little bigger. That said, the only way to really up the ante and throw on some big off-road tires is by making a few changes to the vehicle’s suspension system or wheel setup.
Most off-road enthusiasts make the decision to modify their vehicles to fit bigger tires by installing lift kits, leveling kits, wheel spacers, wheels with different offsets, cutting the fenders, or any combination of these. Installing something like a 3-inch lift kit, for example, makes it relatively easy for a truck that fits 30-inch tires from the factory to suddenly fit 33s or something even bigger.
The good thing about many of these modifications is that they’re relatively easy to do and, in my experience as a technician and enthusiast myself, can be knocked out in a day or less. Few mods make a bigger difference in terms of both looks and performance than installing a lift kit and putting on some big tires, which is why so many people do this to trucks, Jeeps, and other off-road vehicles.
Shop For Off-Road Tires At CARiD
CARiD makes it easy to find a set of off-road tires by having a couple of different ways to search for them. If a vehicle is stock and there aren’t any plans on making modifications like installing a lift kit, simply search by the year, make, and model, and the site will only show tires that closely match the factory-installed size. This takes all the math and guesswork out of it and it’s super easy to do.
On the other hand, if someone just installed a lift kit and wants to throw on a big set of 35s, they can also just search by tire size. This allows enthusiasts to find tires using either one of the sizing conventions we went over in this article, making it a breeze to find the perfect set for their next off-road excursion.
For more help and to find a set of tires that fits as easily as possible, check out the CARiD live chat tool, where experts are standing by 24/7 ready to go.
