Wheels and Tires in Depth

Reading the sidewall

Take the size P215/65R15 95H. The P means a passenger-vehicle tire. 215 is the width in millimeters from sidewall to sidewall. 65 is the aspect ratio: the sidewall’s height is 65 percent of the width. R means radial construction, which gives a flexible sidewall and a stiff tread. 15 is the wheel diameter in inches. 95 is the load index, a code for how much weight the tire can carry, and H is the speed symbol.

More on the sidewall

• Load index is a code number, not pounds; tire charts turn it into a weight. Your owner’s manual or a tire dealer can tell you what your car needs.
• Maximum permissible inflation pressure is the most the tire should ever hold. It is not your car’s recommended pressure.
• Tire Identification Number: the letters DOT, then the plant code and more. The last four digits are the week and year it was made: 0308 means the third week of 2008. It may be printed on only one side.
• Treadwear, traction and temperature grades (UTQG): a treadwear grade of 200 should last about twice as long as 100 under the same test; traction runs AA, A, B, C for stopping on wet pavement; temperature runs A, B, C for resisting heat.

Pressure and TPMS

The right pressure for your tires is the one the vehicle maker lists on the tire and loading information label, on the driver’s door edge or door post, or in the owner’s manual. Check all the tires, including the spare, at least once a month and before long trips, when they’re cold, meaning parked for at least three hours. Pressure changes about 1 psi for every 10°F change in temperature, so tires read low on the first cold morning of fall.

Cars from the 2008 model year on have a tire pressure monitoring system (TPMS), which lights a warning when a tire is significantly under-inflated. It isn’t a substitute for a gauge: by the time it lights, the tire is already well below where it should be. If the TPMS light flashes for 60 to 90 seconds and then stays on, the system itself may have a problem. The step-by-step check is in Check Tire Pressure and Tread.

Tread depth and age

Tires should be replaced when the tread is worn to 2/32 inch. Built-in wear bars in the grooves show when you’ve reached it, and the penny test works too: put a penny head-first into a groove; if you can see the top of Lincoln’s head, it’s time. Tires also age. You can’t see aging just by looking, and some vehicle and tire makers recommend replacing tires 6 to 10 years old regardless of tread.

Wear patterns and what they mean

• Center worn, edges fine: too much air; the tire bulges in the middle.
• Both edges worn: too little air; the sides sag and carry the load.
• One edge worn: the wheels are out of alignment (often camber).
• Feathered, saw-tooth edges: toe is off.
• Random patchy spots: wheels out of balance, or worn shocks or ball joints.

Rotation, balancing and alignment

Rotating the tires, moving them front to back and side to side, evens out wear. If your manufacturer recommends it, the usual interval is every 5,000 to 8,000 miles, or sooner if you see uneven wear; some vehicles, such as those with different front and rear sizes, can’t be rotated. Balancing adds small weights to the wheel to cancel heavy spots so the wheel doesn’t shake; new tires should always be balanced when installed. Alignment sets the wheel angles (Lesson 9).

Check yourself

Tap an answer to check it, or open “Show the answer.”

1. In P215/65R15, what does 65 mean?

   • The tire is 65 inches tall
   • The sidewall height is 65 percent of the width
   • It holds 65 psi
   • It’s rated for 65 mph
   Show the answer

   The sidewall height is 65 percent of the width. 65 is the aspect ratio: sidewall height as a percentage of the tire’s width.

2. Where do you find the right pressure for your tires?

   • The maximum printed on the sidewall
   • The label on the driver’s door edge or post, or the owner’s manual
   • Any gas station sign
   • The TPMS light
   Show the answer

   The label on the driver’s door edge or post, or the owner’s manual. Use the vehicle maker’s figure. The sidewall number is the maximum the tire can hold, not the target.

3. A tire is worn in the center but fine at the edges. What’s the likely cause?

   • Too much air
   • Too little air
   • Bad toe
   • A heavy spot
   Show the answer

   Too much air. Too much air makes the tire bulge in the middle, so the center wears first.

4. A tire’s DOT code ends in 4521. When was it made?

   • April 2021
   • The 45th week of 2021
   • The 21st week of 2045
   • It expires in 2045
   Show the answer

   The 45th week of 2021. The last four digits are week and year: week 45 of 2021.

5. Your TPMS light comes on. What does it tell you?

   • All tires are perfect
   • At least one tire is significantly under-inflated
   • The tires need rotating
   • The spare is flat
   Show the answer

   At least one tire is significantly under-inflated. TPMS warns only when a tire is well below the right pressure. Check all four with a gauge.

Go deeper

• NHTSA: Tire Safety Ratings and Awareness NHTSA’s tire safety page: pressure, tread, aging and recalls.
• NHTSA: Tire Safety: Everything Rides On It (DOT HS 809 361) NHTSA’s booklet on reading the sidewall.
• Being an Adult 101: Check Tire Pressure and Tread The step-by-step pressure and tread check.
• Being an Adult 101: Change a Flat Tire How to change a flat tire.

Sources

• NHTSA: Tire Safety Ratings and Awareness
• NHTSA: Tire Safety: Everything Rides On It (DOT HS 809 361)
• U.S. Navy, Construction Mechanic Basic (NAVEDTRA 14264A), Chapter 14: Automotive Chassis and Body
• Georgia Consumer Protection Division: Maintaining Your Vehicle's Tires
• Purdue Extension: Tires for the Road and Field
• Southern Illinois University Automotive Technology: Alignment Information and Vocabulary
• NIST Guide to the SI, Appendix B.8: Conversion Factors