Although the primary difference between car and truck tires is size, it's not the most significant. And even by this measure, rim diameter sizes are converging with the introduction of 20- and 22-in. car and pickup/SUV tires.
Load and inflation rating, however, are two very critical factors differentiating car and truck tires; they are the most important parameters in determining a tire's duty cycle capability. For passenger car tires, the mandatory sidewall stamping typically reads: Maximum Load x lb. @ y psi Maximum Inflation Pressure. Truck tires typically read: “Maximum Load x lb. @ y psi. This is a subtle but very significant difference.
For car tires (designated “P”), MAXIMUM Load capacity and MAXIMUM cold inflation pressure limits are always stated and should not be exceeded. Load adjustments may also be required.
For truck tires, only the load capacity is stated as a maximum. The cold inflation pressure stated is the minimum acceptable pressure corresponding to that maximum load at normal highway speeds. It's not a maximum value and should be adjusted according to tire makers' recommendations for different axle positions, wear patterns and maintenance practices.
The inflation pressure details on tire sidewall stamping can be informative but are not always the same as application-specific recommended inflations for individual service conditions. Users should follow the specific inflation recommendations and load restrictions given by the vehicle OEM, provided they don't conflict with tire maker recommendations or tire industry standards. For clarification, contact The Tire and Rim Assn, www.USTRA.org or e-mail [email protected]; or the Rubber Manufacturers Assn., www.rma.org.
Inflation recommendations, including those stated as “Maximum,” are cold pressure values — taken with tires at the prevailing ambient temperatures — and do not include any pressure buildup due to vehicle operation.
Inflation pressures, as well as loads, can significantly affect vehicle handling and stability. This is more evident in passenger cars and other light-duty applications, as they are usually designed to operate at lower inflation pressures than truck tires. Typical passenger tire pressures range from about 26 to 41 psi, compared to large OTR truck tires that operate in the 100-psi range. An underinflation of 5 psi, for example would be a 17% underinflation of a tire with a 30 psi target, compared to only 5% underinflation if the optimal inflation were 100 psi If “P” tires are used in light truck, trailer, or multipurpose passenger vehicle applications, a load service factor of 1.10 applies, i.e. the “P” tire selected must be rated to carry at least 10% more load than if it were being used on a passenger car.
Another caution applies to vehicles subject to asymmetric loading, e.g., those equipped with heavy on-board servicing or loading/unloading equipment, extra fuel tanks, etc. Tire inflations should cover the heaviest loaded wheel position across each axle, which can be determined only by weighing each axle end. It's not acceptable to weigh the axle and average the load equally among the tires across the axle.
In real-world service, maintenance practices influence tire operating inflation pressures and, thus, tire performance. Generally, longer intervals between tire service/maintenance should be correlated with higher tire reserve load capacity. Since tire load-carrying capability at any given inflation is fixed, fleets with longer intervals between tire checks should increase target inflation pressures within guidelines endorsed by tire suppliers.
