A string like 11R22.5 148/145L stamped into the rubber tells you almost everything you need to replace a heavy truck tire correctly. The trouble is that most drivers glance past it, order whatever the last tire was, and hope it matches. Each character in that sequence carries a separate meaning, and getting one wrong can mean a tire that rubs the frame, carries too little weight, or wears out months early.
What Sidewall Codes Mean
Take a common size such as 11R22.5. The first number, 11, is the nominal section width in inches for this older measurement style. On metric-sized tires you will instead see a three-digit millimeter width followed by an aspect ratio, like 295/75R22.5, where 75 means the sidewall height is 75 percent of the tread width. The letter R confirms radial construction, which is standard on almost every modern highway truck tire. The final number, 22.5, is the rim diameter in inches the tire is built to fit.
Beyond the size block you will find a maze of smaller stamps: the DOT code with the manufacturing date in its last four digits, the tread wear and traction grades on some tires, the maximum inflation pressure, and the ply or construction rating. Reading them in order keeps you from mixing up width and diameter, a mistake that leads to tires that physically will not mount.
Reading Load Ratings
The pair of numbers after the size, such as 148/145, is the load index, and it is where many buyers get caught. On a dual-capable tire the first figure is the load index for single application and the second for dual application, because a tire mounted in a dual set carries less per tire than one running alone. Each index number corresponds to a specific weight in a published chart; 148 translates to roughly 3,000 kilograms per tire in single use. The letter at the end, the L in 148/145L, is the speed symbol, telling you the maximum rated speed for that load.
Matching the load index to your axle weights matters more than matching the brand. A tire that fits the rim perfectly but carries too little weight will run hot and fail under a loaded trailer. Before ordering a set, suppliers such as Pneus Québec will cross-check the load index against the position you are fitting, since a steer tire, a drive tire, and a trailer tire often need different ratings even when the size is identical. For operators running mixed loads across Quebec’s long highway routes, that single check prevents the most expensive kind of guesswork.
Keep the inflation pressure stamped on the sidewall in mind too. The maximum pressure listed is the figure at which the tire achieves its full rated load. Running below it does not just soften the ride; it lowers the weight the tire can safely carry, which effectively cancels out the load index you paid for.
Where Markings Mislead
The biggest trap is assuming the sidewall describes the tire’s current condition. The DOT date code tells you when the tire was built, not when it was sold or installed, so a tire that looks new may have spent years in a warehouse. Rubber ages whether or not it turns, and a date more than a few years back is worth questioning.
Section width is another soft number. The stamped width is measured on a specified reference rim, and mounting the same tire on a wider or narrower wheel changes its actual footprint. Two tires marked the same size can sit differently on your equipment depending on the rim beneath them.
Finally, remember that a matching size does not guarantee a matching tire. Tread pattern, casing design, and position-specific engineering are not fully captured in the sidewall code, which is why a steer-position tire and a trailer tire can share 11R22.5 and behave nothing alike on the road.
The next time you are near a parked truck, crouch beside one tire and read its sidewall from left to right until every stamp makes sense; once you can decode one, matching your next replacement becomes a two-minute job instead of a gamble.