Do we really need this much information?
Cold winter nights offer the best opportunity for many of us to catch up on reading material. Whether it’s job-related trade journals, industry studies, or a good novel, there is much information and entertainment available in print.
Chances are pretty good, however, that unless you’re an incurable insomniac, you haven’t squandered time reading Title 49 of the Code of Federal Regulations. That’s the government publication that chronicles, among other things, the rulemaking processes of the National Highway Traffic Safety Administration (NHTSA). Part 119 of the Code specifically details labeling and laboratory test standards for new highway tires designed for vehicles other than passenger cars. Doesn’t that just get your heart racing?
Let’s Go Metric
During the last several years, the federal government has been methodically updating regulations to require all measurement units conform to the SI (metric) system, and FMVSS 119 is now receiving this attention. NHTSA’s final rule, published in May 1998 requires all FMVSS 119-covered tires manufactured on or after May 27, 2003 to be permanently labeled with dual SI/English stamping.
Some manufacturers have been using newly stamped molds since mid-1998. All remaining tire molds, including those used to manufacture truck, bus, trailer, and motorcycle tires, must be removed from service and reworked prior to late May 2003. This will require significant effort and expense on the part of all tire manufacturers.
New Dual Markings
Tires for passenger cars covered by the FMVSS 109 standard have been manufactured with dual load and inflation sidewall stamping for years, dating back to the introduction of P-metric tire sizing. The SI system (Systeme International d’Unites) was originally proposed in international circles as a modernized version of traditional metric units, and has been promoted in North America by the ASTM (American Society for Testing and Materials). SI measurement units are kilograms (kg) for load and kilopascals (kPa) for inflation pressure. This differs from the traditional metric unit of bars for inflation pressure.
Most North American truck and trailer tire manufacturers, working through the Tire and Rim Association, also plan to add European style load index and speed ratings at the same time their molds are reworked to include the SI units.
The following example shows a typical comparison of the stamping chosen by one major tire maker.
Note that the SI units are required to be primary with the English units following in parentheses. Because European load ratings and the loads defined by the U.S.-based tire size/load range system often don’t match exactly, some tire load ratings are also being revised. Generally, only dual tire loads are affected and nearly all revisions call for marginally higher load ratings. Some inflation pressures will also be increased.
Current Products OK
Tire engineers we contacted said their current products will meet the higher load ratings without needing structural revisions. They recommend that when tires with old and new markings are mixed, you should only inflate or load tires to the maximum of the least-rated tire on the axle.
They further caution that when tires are properly applied in dual pairs, inflation pressures should be equalized, regardless of any differences in the sidewall load/inflation stamping. These are important considerations given that many tires with the older English-only markings will be in service for years, both as low mileage original treads and as retreaded casings.
We do not know of any industry bulletin currently available covering these practices. Therefore, contact your tire suppliers for specific recommendations of application and maintenance guidelines when mixing tires with old and new markings.
What It All Means
In the example of new stamping, refer to the line directly under tire size, which reads “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 at a load index of 141 in dual applications.
These service descriptions, common for the European market, can be added, but are not required. The speed rating inclusion is optional and there is no standardized test procedure or FMVSS 119 requirement for high speed testing, except for motorcycle tires and 14.5-inch rim diameter or smaller tires in load ranges A-D. However, tire makers may include all of this information so that the same molds and inventory could be used for several international markets.
For the present, it appears these mixed information tires with different load or inflation ratings shouldn’t pose a problem for tire dealers or end users. Vehicle manufacturers may choose to revise GAWRs (gross axle weight ratings) upward on selected trucks or trailers once their tire suppliers have completed the conversion to new load ratings. This may happen on vehicles where tires are now the limiting factor.
For example, the new dual load for an 11R22.5 load range G increases 90 lbs per tire beyond the old rating, which allows the GAWR to be raised 360 lbs on an axle fitted with dual tires. If this revised rating occurs, replacement tires will 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 on-road inspection or weigh stations.
Under One System
Although the new SI units are expected to be globally accepted and will help unite global tire nomenclature, don’t expect any changes in truck tire maintenance or a 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 North American standard practice. The tire guys just wish that all end users would measure their tires more often and use accurate gauges regardless of the system of units.