Inflation and load ratings will be going up for some dual tires
Beginning May 27, 2003, new nonpassenger-car pneumatic highway tires must be stamped with measurements in both SI and English terminology. In the SI (Systeme International d'Unites) metric system, load is expressed in kilograms (kg) and inflation pressure in kilopascals (kPa). Most tire makers also plan to add European-style load index and speed ratings.
Some tire load ratings are also being revised, since European load ratings and the loads defined by the U.S. tire-size/load-range system often don't match exactly. In general, only dual tire loads will be affected, with most revisions calling for marginally higher load ratings. And inflation pressures will also be increased in some cases. Current tires are expected to meet the higher load ratings with no structural revisions.
Following is an example of the stamping chosen by one manufacturer:
Max. Load Single 2800 kg (6175 lb.) at 760 kPa (110 psi) cold
Max. Load Dual 2575 kg (5675 lb.) at 690 kPa (100 psi) cold
Tread 5 plies 4 steel cord + 1 polyamide cord
Sidewall 1 ply steel cord
Load Range G
However, engineers recommend that when tires with old and new markings are mixed, they should only be inflated or loaded to the maximum of the least-rated tire on the axle. And when tires are properly applied in dual pairs, inflation pressures should be equalized, regardless of any differences in the sidewall load/inflation stamping. Contact your tire suppliers to obtain specific recommendations when mixing tires with old and new markings.
These are important considerations given that many tires with the older English-only markings will be in service for years to come, both as low-mileage original treads and as retreaded casings.
Let's look at the new stamping example, specifically the line directly under tire size: “144/141 L.” The “L” indicates the tire is rated for speeds up to 120kph (kilometers per hour) at a load index of 144 in single applications or 141 in dual applications.
The speed rating is optional and there's no standardized test procedure or requirement for high-speed testing, except for motorcycle tires and 14.5-in. rim diameter or smaller tires in load ranges A-D.
It remains to be seen whether OEMs will revise GAWR's upward on selected vehicles once tire suppliers have completed the conversion to new load ratings. This could happen when tires, rather than axles, springs, etc., are the limiting factor. For example, the new dual load for an 11R22.5 Load Range G increases 90 lb. per tire beyond the old rating, allowing the GAWR to be raised 360 lb. on an axle fitted with dual tires. If this new rating occurs, replacement tires would have to be selected from those having the new stamping, unless another type of certification were obtained from the manufacturer. Even then, operators could face enforcement problems at weigh stations.
Revised GAWR may also result in higher GVWR, which could push a new truck over a significant value (such as 25,999 lb. or 32,999 lb.) for classification purposes. If you're buying a new truck, discuss this with your dealer.
Although SI units are expected to be accepted globally, don't rush to purchase new tire gauges calibrated in kPa. A quick survey of Canadian and Mexican tire dealers and fleet operators indicated they all rely on measuring tire pressure using psi, making this a standard practice in North America. The tire guys just wish all end users would measure their tires more often and use accurate gauges, regardless of the system of units.