Calculating tire sizes can be a real mystery.
Hopefully this article will help you get a better understanding of how it all works.
Below is a picture of the 3 things you will need to figure out how big your tire should be.
Tire width, Aspect Ratio, and rim diameter.
Keep in mind that tire manufactures can vary on sizes.
So far I have found some automobile manufactures off by as much as 1/2" in overall tire diameter.
However, most of the dirt bike tires I have checked are very close.
When you look at tire sizes you will see something that looks like this: 80/100-21.
What exactly do those numbers mean?
The first number 80 is the width of the tire in millimeters.
The second number is a ratio of the tire width called the Aspect Ratio.
In this example the second number is 100, so 100% of the tire width (80mm) is also the tire aspect ratio, 80mm.
The last number 21 is easy, the rim height is 21" in diameter.
To calculate the overall tire diameter you will need to convert mm in inches, or inches into mm depending on where you live
Since we are in the USA we will be using inches
Take the aspect ratio 80 and multiply by 0.03937 and you get 3.1496".
Take the aspect ratio 3.1496" and multiply by 2 and you get a combined aspect ratio of 6.2992".
Last, add the rim diameter of 21" to the combined aspect ratio of 6.2992" and you get a overall tire diameter of 27.2992".
Now, take a look at the picture below and see if this example makes sense:
But lets say that the tire is 80/75-21
