Dirt Bike Tire Size Meaning – Tire Size Notation Explained

Written by

James Stevens

Fact-checked by

Robert A. Verdin


If you check the tires of your dirt bike, you’ll see a bunch of numbers and letters that may make you wonder if they mean anything.

Those numbers’ meanings indicate various pieces of information, including the tire’s width, aspect ratio, and rim diameter. Aside from that, there are other numbers that show its load/speed rating, as well as a sign specifying the direction it rotates.

It’s important to know what the dirt bike tire size meaning when looking for a tire replacement. Without these tires specifications, you’ll end up with incompatible products that may affect your bike’s performance.

Tire Size Notation Explained


Among the things you have to consider when buying a new tire for your dirt bike is the tire size, this includes factors such as tire width, rim diameter, or aspect ratio. Let’s look at what these numbers mean in this tire size guide.

Front Tire Size
Metric Alpha Inches
60/100 90/80 2.50/2.75
70/100 90/901 2.75/3.00
80/100 100/80 3.00/3.25
Rear Tire Size
80/100 80/90 2.50/3.60
90/100 110/90 3.60/4.10
100/100 120/80 4.00/4.10
110/100 130/80 4.00/4.50
120/100 140/80 5.00/5.10

To make it easier to follow, we’re going to refer to the dirt bike tire size chart above.

1. Tire Width


First, the first three numbers in the sequence of code stamped on your tire refer to its width. These numbers represent the nominal width of the tire, which is derived from the distance between the furthest part of a sidewall to the other.

Tire width is normally shown in millimeters. So, for instance, if the code on your tire reads P215/65R1589H, it means the product’s width is 215 mm.

2. Aspect Ratio


After the three numbers indicating the tire width is the slash, followed by the aspect ratio—a number in tire size that represents the sidewall height.

However, it’s measured in percentage of the tire’s width. Let’s use the same code above—P215/65R1589H. In this case, the aspect ratio would be 65, meaning that the sidewall height can be calculated as follows: 215 mm x 65% = 140 mm.

This is important to consider since different motocross tires have different sidewall heights, even if they share the same aspect ratio. For the most part, tires with larger aspect ratios tend to be taller than those with smaller aspect ratios.

3. Tire Construction


Unlike the rest, tire construction is represented by a letter instead; some common markings you may see are:

  • B (Bias Belted): tires with belts added under their tread and have lower rolling resistance.
  • R (Radical): tires with reinforced steel cords or belts. They have a good grip on wet roads and can run very fast.
  • Dash symbol (Bias-Ply): tires with nylon or Kevlar cords woven in a crisscross pattern at a 45-degree angle. They have strong sidewalls, can handle heavy loads, and make good off-road dirt bike tires.

In the case of a tire with the code P215/65R1589H, you can see that it contains the letter “R”. This means the tire has radial construction.

4. Rim Diameter


If the code doesn’t contain the construction type, the two numbers after the aspect ratio indicates the rim diameter in inches. In some cases, you may see a hyphen separating the two pieces of information, such as: M401 80/100-21 51M (2912). In this case, the rim diameter would be 21 inches.

For codes with construction type, the rim diameter would follow the letter. As an example, the code P215/65R1589H indicates its diameter is 15 inches.

5. Load/Speed Rating

Tire Size

Load Index Speed Rating Max Load (kg)

Max Speed (km/h)

80/100-21 51 M 195 130
90/90-21 54 R 212 170
100/90-19 57 M 230 130
110/90-19 62 M 265 130
120/80-19 63 M 272 130
120/90-18 65 R 290 170
140/80-18 70 R 335 170

Lastly, we have the number that indicates a tire’s Load/Speed Rating. Now, there are lots of numbers and letters used to indicate a tire’s Load/Speed Rating, so we highly recommend referring to a comparison chart with this information included.

The number will indicate the total weight a tire can support. Meanwhile, the letter specified on the tire will dictate the maximum speed it can reach.

Using the example above – M401 80/100-21 51M, we have a load/speed rating of 51M, which means the tire can handle a maximum weight of 430 pounds and reach a maximum speed of 81 mph.

Importance of Choosing the Right Tire Size


Now that you have an idea of how to read a dirt bike tire size, let’s now briefly talk about why choosing the right tire size is important.

Traction and stability are among them, though the size of your dirt bike’s tires can also affect aspects like acceleration, handling, and braking.

Basically, the bigger your tire, the more grip it has on the road, which allows for easier handling and shorter braking distance. Meanwhile, smaller tires can accelerate much faster since they have smaller resistance.

Note that when looking for a specific dirt bike wheel size, you have to take into account size conversions as well. To be clear, older tires use the imperial system, while the newer ones use metric sizing.

Frequently Asked Questions

What’s the average tire size for most dirt bikes?

Most full-sized dirt bikes on the market use an 18-inch tire for the rear and a 21-inch tire for the front.

For mini bike tires, however, the size ranges from 4 inches to 12 inches.

How do I manually measure the size of my tires?

If, for some reason, you can’t read the codes on your dirt bike’s tires, you can instead use a calculator and a ruler to measure them manually.

Start by taking your tire’s width by measuring its edge width. After that, measure the sidewall and divide the edge width by the length of the sidewall. Next, multiply by 100 to get the aspect ratio.

Lastly, take the rim diameter by measuring the tire’s inner diameter.


Now that you know about the different dirt bike tire size meaning, it will be much easier for you to find the right tire size for your dirt bike.

Fortunately, the formatting of these numbers is pretty much the same across different dirt bike brands and models. But if you want to be sure, you can manually measure your dirt bike dimensions.

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James Stevens

James Stevens is an expert bike mechanic who knows everything from basic repairs to custom modifications. What sets him apart is his ability to explain complex concepts in a way that's easy to understand. Check out his content on Speedway if you need help with upgrades or modifications for your bike.