How to Read the Sidewall of a Tire

Have you ever taken a close look at the sidewall of your car’s tires? Those numbers, letters, and symbols actually contain a wealth of important information that any driver should understand. Learning to decode all the markings on a tire’s sidewall empowers you to make smarter decisions when purchasing new tires or maintaining your existing ones.

Here’s a quick step-by-step guide:

  1. Look for size codes like P215/65R15 – gives width, aspect ratio, type (P is passenger car), rim diameter.
  2. Check ratings – 95 load index (max weight capacity), H speed symbol (max speed).
  3. See DOT certification markings and serial number with mfg date.
  4. Identify materials e.g. polyester cords, steel belts.
  5. Compare grades for treadwear, traction, and temperature performance.

Overview of Key Elements on a Tire Sidewall

The sidewall of a tire contains standardized labelling that reveals many technical specifications and properties at a glance. These include:

  • Size designations – width, aspect ratio, construction type
  • Service descriptions – load index, speed rating
  • Certification markings
  • Material information – type of cord, number of plies
  • Performance grades – treadwear, traction, temperature
ElementDetails Encoded
SizeWidth, aspect ratio, rim diameter
Service desc.Load capacity, speed capability
CertificationDOT safety standards
MaterialsCord types, ply composition
PerformanceWear, grip, heat range

Familiarizing yourself with these key items enables you to compare different tire models quickly and make better decisions.

Decoding Passenger Tire Sizing

Let’s break down a sample tire size marking on a common passenger tire, the P215/65R15 95H.

P = Passenger vehicle tire 
215 = Section width in mm
65 = Aspect ratio (% of section height to width)
R = Radial construction
15 = Rim diameter in inches
95 = Load index (load carrying capacity)
H = Speed rating 
  • The section width indicates the full width across the tire tread in millimeters when correctly inflated.
  • The aspect ratio gives the sidewall height as a percentage of the width – useful for estimating overall diameter.
  • Radial denotes the common type of ply construction used today with cords running perpendicular to the tread.
  • The rim diameter must match your vehicle’s wheel rim size.
  • The load index is a numerical code (95 = 1521 lbs) that grades how much weight a tire can safely carry at speed when fully inflated.
  • The speed rating symbol (H = 130 mph) denotes the maximum safe speed that the tire can handle under load.

So for our example, we can determine that:

  • It fits 15″ wheel rims
  • It has a 215mm tread width and 139mm sidewall height (= 215 * 0.65)
  • It can bear 1521 lbs per tire at the car’s top speed
  • It can safely sustain speeds of up to 130 mph

These measurements tell us the precise dimensions and capabilities we need to match to our vehicle.

💡Pro tip: you can easily compare various tire metrics using the sizing codes rather than actual dimensions.

Locating Tire Certification & Materials

Every street legal tire sold in the US carries standard DOT certification markings on the sidewall. Look for the “DOT” label which may be followed by an identification or serial number acting as a tracking code. Adjacent to this is typically a 4 digit manufacturing date code indicating the week and year the tire was produced.

You can also find details on the cord material and number of plies in the tire carcass. Modern radial tires often use polyester or steel belt cords below the tread, while cheaper bias ply tires have diagonal nylon or polyester plies. More plies generally confer higher load capacity and stiffness – but increased cost and weight.

Here’s an example showing the certification & construction:

Polyester + 2 Steel Belts 
2 Polyester Sidewall Plies

This tire was made in the 22nd week of 2021, has 2 steel belts under the tread plus 2 sidewall plies, all using polyester cords.

Understanding the materials helps ascertain quality and capability tradeoffs when selecting replacement tires. Steel belted radials offer the best strength, handling and wear life.

Locating Performance Grades

The DOT mandates passenger tire manufacturers to grade tires on three key performance parameters on a scale from 1 to over 500:

  • Treadwear – Relative wear life (higher = longer lifespan)
  • Traction – Braking grip capability (higher = shorter stops)
  • Temperature – Heat endurance at speed (higher = safer)

So a grading like “200 A A” means the tire has excellent dry pavement braking traction and heat resistance, with reasonably long tread life – a balanced all-rounder.

Selecting tires specialized for your needs is easier when taking advantage of these standardized grades during your research process. High performance sports models tend to emphasize traction and heat resistance over tread life.

UsagePriority Metrics
Family vehiclesTreadwear & Temperature
Sports carsTraction & Temperature
Off-road/SnowTreadwear & Traction

Matching Tires to Wheels

In addition to all the specifics encoded directly on the tire sidewall, you must match some physical properties to your vehicle’s wheels:

  • Rim diameter – Must exactly match wheel rim size
  • Rim width range – Should fall within recommended min/max
  • Load index – Must meet vehicle’s weight/load requirements
  • Speed rating – Should meet vehicle’s max speed
  • Overall diameter – Must have sufficient clearance

Using the industry standard sizing, ratings and material info allows rims and tires to be interchangeable across makes and models as long as they meet the above criteria.

If unsure, consult your vehicle owner’s manual or mechanic to get the required tire parameters for your car.

Frequently Asked Questions

What does a missing speed rating or service description mean on a tire?

Older tires that don’t list a speed rating or load index may be generic multi-purpose models with adequate passenger car capabilities. Always verify with manufacturer specs if concerned.

How do I calculate overall tire diameter from the sidewall size codes?

Take section width in mm, multiply by aspect ratio as a decimal, and add rim diameter in inches. For our example 215/65R15 tire:

Diameter = Width x Aspect Ratio + Rim Diameter  
           = 215 x (65/100) + 15
           = 139.75 + 15
           = 154.75 mm (approx 6.11 inches)

What is the difference between tubeless and tube-type tires?

A: Modern tubeless tires only use an air-tight inner liner instead of needing a separate inner tube. This prevents air leaks when punctured. Older tube-type tires require you to replace the tubed inner core if they get punctured.

Learning to decode the wealth of technical specifications, certifications and performance data on tire sidewalls gives you an advantage in understanding exactly what you are getting. With some practice reading tire markings using the standard systems covered here, you can make enlightened decisions whether replenishing your vehicle’s tires or upgrading to high performance models.

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