The majority of content written on the side of a Dirt Bike Tyre is to help identify what size, model and terrain the tyre is suited to. The first series of numbers you will need to learn are the ones which relate to sizing followed closely by the tread pattern the tyre is suited to.
How to choose the right size motocross tyre:
The sequence of numbers which give you the tyre sizing are set to give you first the tyre width, then the tyre wall height and finally the circumference of the rim the tyre will fit onto. (See example below)
The “110” refers to the width of the tyre in millimeters as does the “90” which refers to the wall height as a percentage of the width of the tyre. The “19” refers to the circumference of the rim which the tyre suits in an inch measurement. (Some variations of this are listed below)
Standard Dirt Bike Tyre Sizes
The majority of full-sized dirt bikes in the world run a 21” front tyre and most motocross bikes run a 19” rear. Trail bikes vary with many running an 18” on the rear to suit the different types of terrain covered in enduro.
Here is a metric/imperial tyre sizing conversion chart to help make the right decision: