Every heavy-truck tire carries its specifications molded directly into the rubber of its sidewall. Those raised letters and numbers are not decoration or branding filler. They are a compact record of the tire’s size, construction, load capacity, and the maximum speed at which it can carry that load safely. Reading them correctly is the difference between a tire that matches the vehicle it is mounted on and one that quietly undermines it.
What Sidewall Codes Mean
A typical commercial tire marking looks something like 295/75R22.5. The first number, 295, is the section width in millimeters measured from sidewall to sidewall. The 75 is the aspect ratio, meaning the sidewall height is 75 percent of that width. The letter R indicates radial construction, which is standard for modern heavy-truck applications. The final number, 22.5, is the rim diameter in inches that the tire is built to fit.
Alongside the size you will find other codes worth knowing. The DOT stamp includes a four-digit date code telling you the week and year the tire was manufactured, which matters when you are evaluating stock that has sat in storage. You may also see a ply rating or a load range letter, along with markings that indicate whether the tire is designed for steer, drive, or trailer positions. Treadwear and traction grades appear on some tires, though position-specific tread design often tells you more about intended use than any single grade.
Reading these codes becomes second nature once you handle enough tires, but the stakes stay high because a mismatch is easy to overlook. Fleet managers comparing heavy-truck tires for Québec fleets often find that two tires with nearly identical size markings still differ in load range and construction, and that difference decides whether the tire belongs on a loaded trailer or a lighter application.
Load And Speed Ratings
Near the size code you will find the service description, usually a number followed by a letter, such as 144/142M. The numbers are load indexes, and most heavy-truck tires carry two because they can be fitted in single or dual configurations. The first index applies when the tire runs as a single; the second, lower number applies when two tires share the load as a dual set. Each index corresponds to a specific weight in kilograms, which you look up on a standardized load index chart rather than reading directly off the tire.
The letter is the speed symbol, and it marks the maximum speed the tire is rated to sustain while carrying its rated load. The letters L through M cover the range most commercial trucks operate within, and a higher letter means a higher permitted speed. The rating assumes the tire is properly inflated and not overloaded, so it is a ceiling under correct conditions rather than a guarantee under abuse.
These two figures work together. A tire can have ample load capacity but a speed rating too low for the routes a vehicle runs, or a speed rating that looks generous paired with a load index that falls short of the axle’s demands. Both numbers have to clear the real requirements of the truck, not just one of them.
Matching Tires To Fleets
The practical task is lining up what the sidewall promises with what each vehicle actually does. Start with the axle loads the truck carries fully laden, add the margin that Québec’s winter conditions and road surfaces justify, and confirm that the load index meets or exceeds that figure in the configuration you will use. Then check the speed symbol against the top sustained speed of the routes the truck runs.
Position matters too. Steer tires, drive tires, and trailer tires are engineered for different forces, and mounting the wrong type in a position shortens its life and compromises handling. Consistency across a dual set is equally important, since mismatched diameters force one tire to carry more than its share. For a fleet running mixed vehicles across Québec, keeping a simple record of the required size, load index, and speed symbol for each position removes guesswork at replacement time.
Before your next order, pull the current sidewall markings off one tire in each position of a representative truck and write them down; that short list becomes the reference that keeps every future replacement correctly matched.