Most passenger tires today have metric or “P-metric” sizes. These are the sizes most of you are probably familiar with, such as 205/55R16. But there’s another system of sizing out there, LT Sizing, used primarily for light truck and off road tires.
LT sizes can be confusing because they’re so different from the sizes you’re used to seeing. Well, we’re here to take away some of the confusion, so let’s get right down to it:
What is an LT Tire Size?
The first thing you should know about LT sizes is it has many names. You can hear it referred to as LT, Numeric, Wide Base or Floatation sizes. We’ll get into the reasons for this in just a bit. The second thing you should know is they’re measured in inches, so this isn’t going to be the same thing as a metric size with an “LT” in front of it. In fact, in this case the “LT” notation usually follows the size instead of preceding it.
Right now, you may be thinking “Sure, it’s written in inches, but it still tells you how big the tire is. So how is it different?” Let’s take a look at one and I can break it down for you:
LT Tire Size Breakdown
35: The first number in these sizes is the tire’s overall diameter, or how big around it is when inflated (but not under load). In this case, it’s 35 inches.
12.50: The second number in the size, represents the section width, or how wide the tire is from sidewall to sidewall. (This is what the first number in metric sizes tells you). In this case, it’s 12 and a half inches wide.
24: Just as in metric sizes, the third number represents the rim or wheel size in inches, in this case, 24 inches.
Why are LT Tire Sizes Used?
Typically, this was used on earlier model tires and commercial tires. It’s still used in a lot of commercial tires now, but it’s also used for off road tires, heavy cargo or trailer tires, or tires with higher sidewall ratios than standard. This is part of why this type of size has so many different names.
- Numeric is for tires that are used for heavy cargo or towing, like a trailer tire.
- Wide Base is for tires that have a 16.5 inch rim or higher or are higher (so these would fall under the higher sidewall ratio).
- Floatation generally refers to off road type tires that are designed to drive on loose surfaces like sand or dirt (essentially they “float” on top of these surfaces).
This type of size measurement is very useful for these types of tires. Having the diameter lets you figure out how much clearance the tire will have in your wheel well. This is especially handy with the off road tires, as they tend to have a deeper, chunkier tread than other tires. This is also why it’s important to understand the overall diameter or the number measured in the first part of the tire size.
You see, overall diameter measures the diameter of the tire after it’s inflated but before it’s put on a vehicle. This means that the tire is on a rim and inflated to pressure, but without any weight on it. It also means that the overall diameter will be different from the diameter of the tire when it’s not inflated. It’s an important distinction because you might get a tire that says 35 and measure it, uninflated, around 33 or 34 inches and think you have room to work with when you actually don’t.
How to Measure LT Tire Size
So what if you want to put these tires on your car? Or, if you already have them, what if you want to upsize them? Or what if you have these tires and want to switch back to metric? You could use a converter like this or you can even find a chart like this one:
However, it’s still a good idea to have at least a basic understanding of how to do it yourself. That way you can still figure it out if you’re stuck without a converter and you can tell if the converter you’re using is a bit off.
So, whether you’re upsizing or switching to LT sizes, the first thing you’ll need to do is calculate your tire’s overall diameter (or OD). For those math phobes out there, don’t worry! It’s not that complicated! To get started let’s use size from the chart above, we’ll go with 295/30R26.
Calculate the Sidewall Height of Your Tire
For this step you’re going to take the section width (the first number in the metric size) and multiply it by the aspect ratio (the second number). Now it’s important to remember that the aspect ratio is a percent, so in this case it would be 295x30% or 295x.30 which comes out to 88.5 millimeters
Convert the Sidewall Height to Inches
This part is easier than it sounds. First you’re going to take what you just calculated and multiply that by 2, so 88.5x2=177. Then you’re going to take the result of that equation and divide it by 25.4, so 177 / 25.4=7. This gives you the sidewall height in inches, but we’re not done yet!
Add the Wheel Size
The wheel diameter is already in inches, so you just need to add the two together: 7+26=33, and there you have it!
Your OD is 33inches!
Now, not all numbers are going to convert so cleanly, so occasionally you’ll see something like 31.4 on the chart above. You may not be able to find these fractions in actual tires, so you can round up or down to the closest whole number. Just make sure you have enough clearance in your wheel well (if you’re not sure, I’d recommend rounding down).
So that’s the first part of your size in inches, and you already have the wheel diameter. All you need to do to get a full LT size is convert the section width into inches. Luckily, you already have all the information you need! You just take the first number of your metric size and divide by 25.4. So, in this case it would be 295 / 25.4=11.6.
So, you have your full size: 33x11.6R26
Calculating LT Tire Size Isn't That Difficult
If you want to figure out a metric size from your light truck tires size, you’d basically do this backwards. To convert the section width to millimeters, just multiply by 25.4, then do the other steps in reverse (subtract the rim size from the diameter, multiply by 25.4, divide the result by 2 and then divide by the section width in millimeters to get your aspect ratio). It’s a little less exact going backwards, and you will likely have to round to the nearest whole numbers again.
Even if you don’t use it, you now know how it works, and how the sizes relate to each other. This is good knowledge to have, even if you just use a converter, because you’ll not only be able to tell if something’s off, but you’ll have a better idea of what tire size can work for you.