Two types of the tyre size designation can be found on a tyre sidewall, a tyre width expressed in mm and the tyre width expressed in inches.

1. The tyre width expressed in mm

All tyres for the commercial vehicles must be marked with a Service Description located after the size designation (European regulation ECE-R54). The service description consists of load index and speed rating (e.g. 156/150 L).

Listed below are a few examples of the truck/bus tyre size designations.

Here is an example of the tyre size designation for commonly used truck tyre:

Truck tire size designation

Designation: 315/80R22.5 156/150 L (154/150 M)

The designation meaning:

315
The tyre width (the nominal section width) expressed in mm.
80
The aspect ratio (the ratio of sidewall's section height to the tire's section width) in % (80%).
R
The construction code (the radial construction).
22.5
The rim diameter in inches.
156/150
The load index for single and dual fitment.
L
The speed rating - the tyre's maximum speed. See the speed index table.

The load index indicates the maximum tyre load capacity for single and dual fitment. The first number: 156, means that the maximum tyre load carrying capacity is 4000 kg for the single fitment. The second number: 150, indicates the maximum load carrying capacity of 3350 kg for the dual fitment. See the load index table.

For e.g., a tyre with the load and speed index of 156 L is designed for the load of 4000 kg and the maximum speed of 120 km/h.

The tyre manufacturers have the option of marking tyres with an additional load/speed index for an alternative higher speed or load. That code may be placed in a circle near basic designation (e.g. 154/150M).

Truck tire size designation additional load speed index

The tyre size presented below is common on the buses:

Designation: 295/80R22.5 152/148 M

The designation meaning:

295
The tyre width (the nominal section width) expressed in mm.
80
The aspect ratio (the ratio of sidewall's section height to the tire's section width) in % (80%).
R
The construction code (the radial construction).
22.5
The rim diameter in inches.
152/148
The load index for single and dual fitment.
M
The speed rating - the tyre's maximum speed.

2. The tyre width expressed in inches

Truck tire size designation

Designation: 11 R 22.5 148/145 L

The designation meaning:

11
The tyre width expressed in inches.
R
The construction code (the radial construction).
22.5
The rim diameter in inches.
148/145
The load index for single and dual fitment.
L
The speed rating - the tyre's maximum speed.

You may look up a speed index and adequate speed rating in the Tire speed rating table.

Visit the page Load index chart and find out what is the load capacity of a tyre, based on its load index.