Federal regulations require specific information to be molded on the sidewall of a truck tire. The size, date of manufacture, and maximum load-carrying capacity are among the mandated markings for all truck tires sold in the U.S.
There are also performance requirements that are not optional. Manufacturers must mold the letters “DOT” on the sidewall to indicate it meets those standards. It’s a self-certification, so all the manufacturer has to do is submit the test data that shows the tire meets the requirements.
Another key piece of information on every tire sidewall is the required inflation pressure to carry the maximum load. Many people confuse this with the maximum inflation pressure, but that is not the case. The load and inflation information on the sidewall reflects the required inflation pressure to carry the maximum load at the maximum speed. Molding the maximum speed on every sidewall is not required by federal law, but the default maximum speed for a truck tire in the U.S. is 75 mph unless designated otherwise. The only requirement applies to tires with a maximum speed of 55 mph.
Speed, load, and inflation pressure are all related. In low-speed operations, the load can exceed what is indicated on the sidewall. Again, in some cases, the pressure doesn’t change, while in others, the higher load at the lower speed requires more inflation pressure than what is indicated on the tire. It’s all about heat. Higher speeds create more flexing, which results in additional heat. The lower flexing at lower speeds generates less heat, allowing the load to increase with or without additional inflation pressure.
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For years, every truck tire has been inflated to what the trucking industry believes is the maximum inflation pressure on the sidewall. In reality, most of those truck tires are overinflated because the maximum load is never applied.
Federal regulation states that the tires on an axle must be able to support the GAWR listed on the certification label. On a 20,000-lb. axle with dual tires, each tire must support 5,000 lb. A standard 295/75R22.5 Load Range G in a dual application has a maximum load of 6,175 lb. at 110 psi. Inflating all four tires to the inflation pressure for the maximum load results in an overinflation condition because the maximum load for the axle is 20,000 lb., regardless of what is indicated on the tires.
That same tire will carry 5,155 lb. at 85 psi, which is enough to support the GAWR. At 85 psi, each tire in a dual application is inflated to the load, resulting in the optimal footprint for maximum treadwear and traction. When the tires are overinflated to 110 psi on a vehicle with a 20,000-lb. GAWR, the footprints will have more pressure on the middle of the tread, which can result in irregular and/or accelerated treadwear.
While federal regulation 571.120 requires tires to have the carrying capacity to support the GAWR, it doesn’t mean the inflation pressure must support the GAWR. On a 20,000-lb. axle that only carries 18,000 lb., the same 295/75R22.5 Load Range G in a dual position can support 4,500 lb. at 70 psi, which is sufficient for the load. There’s no room for error, but it’s perfectly legal and acceptable as long as each tire is inflated to 70 psi.
For fleets, the issue is maintaining inflation pressure. Every tire loses 1-2 psi per month. At 110 psi on a vehicle with a 20,000-lb. GAWR, it can take over a year before the inflation pressure reaches the point where it can no longer support the maximum load for the axle. Underinflation is kryptonite for a pneumatic tire because the additional flexing results in excessive heat, which leads to the breakdown of the bonds between the internal components in the tire. It’s the leading cause of “gators,” so a lot of fleets choose overinflation because the consequences are less severe and do not result in roadside failure.
Selecting the optimal inflation pressure for the tires on a commercial motor vehicle is best determined by working with the tire manufacturer. Factors such as inflation pressure maintenance, load distribution, and vehicle configuration all play an important role in determining the inflation pressure that yields the best treadwear, traction, and performance. Inflating every truck to the maximum pressure for its load may seem like the best practice, but it can increase the cost per mile in the long run.
