Tire Pressure Vs. Load Rating

Under inflated tires flex and become damaged.

Over inflation, under inflation or going over the maximum weight a tire can support is a quick way to endanger yourself. According to Mac Demere of 'Edmunds,' nearly every tire blowout occurs on a warm day, on a straight road, at highway speeds when a tire is under-inflated. Protecting yourself, your passengers and your vehicle starts with understanding what your tires are capable of handling in terms of tire pressure and load rating.

Types

Every legally manufactured tire includes details on the sidewall that tell what it is rated for and capable of handling. Tire manufacturers advise never to exceed the maximum PSI stamped on the tire sidewall. A vehicle placard also suggests tire pressure for best ride experience. Tire load rating indicates the maximum weight a tire can support safely. Each tire includes a number on the sidewall that corresponds with the load index, stating maximum weight a tire can carry.

Identification

The tire pressure rating on the actual tire could look like "Max PSI 35," which would mean the manufacturer suggests never exceeding 35 PSI or it could fail. 'Edmunds' suggests filling tire pressure to the listed PSI found on the vehicle placard within the driver's door jamb or owner's manual.
The tire's suggested load rating for a vehicle is listed on the vehicle placard and in the owner's manual. It is not as easy to identify on an actual tire sidewall. Each tire has tire markings across the sidewall made up of letter and numbers. For example, a tire could read 245/40R19 93W. The load rating comes after the tire size markings, and in this case, is 93. The 93 corresponds to the load index chart, which states that a tire with a 93 load rating can support 1,433 pounds.

Significance

Overinflating a tire results in less rubber on the road, or a smaller contact patch. A smaller contact patch results in less grip while accelerating, turning and braking. Underinflating a tire allows the tire to flex and get hot, and is a leading cause of tire failure. The flexing of an underinflated tire at speed damages the tread, tire bead and tire cord. Overloading a tire increases tire stress and can also lead to tire failure.

Function

The state of a vehicle's tires has a large effect on how a car, SUV, truck, motorcycle or recreational vehicle performs. Staying within the load rating will keep tires within their weight limitations. Keeping tire pressure within a specified PSI allows the tire a complete contact patch, leading to increased grip, wet weather performance and lifespan.

Expert Insight

Check tire pressure at least once per week when the tires are cold (not recently driven on). The load rating and tire pressure markings on a tire correspond to a cold tire. Camith Safety Services, a commercial motor vehicle publication, suggests that for every 10 degrees Fahrenheit, up or down, tire pressure will change 1 PSI.

References

• Edmunds: Tire Failures and Unintended Acceleration
• Consumer Reports: Tire Features
• Discount Tire: Load Index Chart
• Between the Lines: Tire Basics

Resources

• Car and Driver: How to Read a Tire