What the Numbers on a Sidewall Actually Say
Every tyre carries its own specification moulded into the sidewall. A marking like 315/80R22.5 156/150K is not a part number, it is five separate pieces of data. 315 is the nominal section width in millimetres. 80 is the aspect ratio, meaning the sidewall height is 80 per cent of the section width. R means radial construction. 22.5 is the rim diameter in inches. 156/150 is the load index, dual and single. K is the speed symbol. Read in that order, the marking tells you whether a tyre physically fits the wheel, whether it can legally carry the axle load, and how fast it may run while doing so.
Section Width Is Nominal Not Actual
The 315 in 315/80R22.5 is the design section width, not a measurement you can rely on for clearance. The actual width varies with the rim it is mounted on and with the specific pattern. This is why our product pages list section width and overall diameter as separate measured values for every size rather than leaving you to calculate them from the marking. If you are checking twin fitment clearance or mudguard gap, use the listed dimensions, not the nominal figure.
Radial and Bias Are Different Constructions Not Different Grades
R means the casing plies run radially, at ninety degrees to the direction of travel, with a stabilising belt package under the tread. A marking without the R, such as 12.00-20, is a bias or cross-ply tyre, where the plies run diagonally and overlap. Radial generally runs cooler, gives longer tread life and lower rolling resistance. Bias is more resistant to sidewall cutting and usually costs less. Neither is simply better. On a short cycle quarry job on sharp rock, bias is often the correct engineering answer.
Load Index Is the Number That Matters Most
A truck tyre marked 156/150 carries a different load in single fitment than in dual. The first number is the dual value, the second is the single value, and each corresponds to a kilogram figure in a standard table. Our size tables give the actual maximum load in kilograms alongside the inflation pressure it requires, because the load rating only holds at that pressure. A tyre run twenty per cent under its specified pressure is not carrying its rated load, whatever the sidewall says.
Ply Rating Is a Strength Class Not a Ply Count
An 18PR truck tyre does not contain eighteen plies. Ply rating is a legacy strength classification carried over from cotton ply construction, and it survives because buyers and fleets still specify by it. Higher ply rating means a stronger casing rated for higher inflation and load. Fitting a higher ply rating than the axle needs costs money and gives a harder ride without adding useful capacity.
Speed Symbol and the Markings That Follow
The letter after the load index is the speed symbol. K is 110 km/h, L is 120, and the scale continues upward through the passenger car range. After that come the service markings that matter at a border: M+S is applied by the manufacturer based on tread geometry, while 3PMSF, the three peak mountain snowflake, requires the tyre to pass a defined snow traction test. Most European winter regulations name 3PMSF specifically. If your market has a winter requirement, check which of the two the regulation actually cites before ordering.
The DOT Code and Manufacturing Date
Tyres sold into North America carry a DOT code, and the last four digits give the week and year of manufacture. 2324 means the twenty third week of 2024. Buyers importing stock should know this exists, because it is the first thing an experienced customer checks and it affects what you can reasonably hold in a warehouse. Marking requirements vary by destination market, so tell us where the container is going and we confirm with the plant what marking is available on those sizes before the order is placed.
Reading a Size Table Before You Order
Every product page on this site lists the manufacturer size table in full: size, ply rating, load index, speed symbol, standard rim, maximum load and inflation pressure, overall diameter, section width and tread depth. Check three things before ordering. First, the standard rim against the wheels actually on the vehicle. Second, the load at your working pressure against the axle plate. Third, the overall diameter against your clearance and, on a driven axle pair, against the tyre on the other side. Most fitment problems we see are one of those three.
