How to Read Tires Sizes & Sidewall Markings

how to read tire sizes and sidewall marking fully explained

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Those markings on the sidewalls of tires might seem like the kinds of scratches only a calculus major could understand, but they’re really not that complicated. Here’s a look at the various markings and what they mean.

by Kelly Taylor

What do Tire Sizes Mean Exactly?

Here’s a common tire size: P225/65R17.

The first letter, P, means it’s a passenger car tire. Tires for a pickup or some sport-utility vehicles might start or end in LT, meaning light truck.

The 225 is the width of the tire in millimetres from sidewall to sidewall.

The 65 refers to the ratio between the width and height of the sidewall. In this case, the sidewall is 65 per cent of the width, or 65 per cent of 225, or 146.25 millimetres. The R means radial, as in the style of construction, while the 17 means it rides on wheels with 17-inch diameters.

reading-tire-sizes
graphic: Kal Tire

Why it’s Important to Understand the Tire Size

These numbers are important to get right when looking for tires, as a P225/55R17 tire will have a different overall diameter and throw off your speedometer. As well, tires with lower aspect ratios will both ride rougher and increase the risk of wheel damage, as there is less sidewall to absorb bumps. 

As an example of a performance tire size, the Porsche 911 Turbo S Coupe takes 305/30R20 on the rear, or 305 mm wide, 30 per cent aspect ratio (sidewalls are only 91.5 mm high), on 20 inch wheels.

Here’s another great piece on how to read your tire from the team at Edmunds.

Sidewall Markings: 3 Popular Types

Tire sidewalls don’t all look the same either. For example, Goodyear Tires offers 3 types of sidewall styles for their products:

  1. Black: all black sidewall with black lettering
  2. Whitewall: white bands on the sidewall vary in width
  3. Letters Outlined: letters on the sidewall are outlined in white

tire sidewall markings 3 types offered by goodyear

Tire Load Index Explained

This number can range from 75 to up 120, and those and each number in between correspond to the maximum load a tire can support when properly inflated.

For example, the tires on our Mazda CX-5 (where do you think I got the P225/65R17 from?), have a tire load index of 100, which translates to 1,764 pounds (on these tires, it was rounded down to 800 kilograms), which, when multiplied by four tires, leaves plenty of headroom for cargo and people. (The curb weight is 1,659 kilograms.)

Each increase by one in the tire load rating (from 100 to 101, for instance) increases the load capacity by 55 pounds (almost 25 kilograms).

Tire Load Index Chart

tire load index chart

What Does Speed Rating Mean & Is It Important? (Yes)

Tires are under extreme amounts of force, not only in holding up the vehicle above but, at speed, there are also extreme amounts of centripetal and centrifugal force acting on the tires. The centripetal force refers to the forces acting inline with the arc of rotation of the tire, while centrifugal force refers to the forces pushing away from the centre of the tire.

Both forces are central to a tire’s speed rating: too weak a tire could tear itself apart at too great a speed. The tire speed rating is a letter, ranging — non-linearly — from Q to ZR. Q rated tires specify a maximum 99 miles per hour. Fortunately, we don’t see too many of those in passenger-car applications. More likely is H, or 130 miles per hour. V rating means 149 miles per hour while Y equals 186 miles per hour.

What you do need to know, however, is that you shouldn’t mess around with speed ratings. If your car has a V-rated tire, don’t replace it with an H-rated tire.

Above 186, it gets a bit more complex, with ZR usually applied to such tires, but the ZR has to be accompanied with the Y. These tires typically have ratings above 186 miles per hour (300 km/h).

The other factor that speaks to speed rating is the tire’s tread, in its ability to bring the car to a halt from such high speeds. Since these higher speeds are typically only seen on racetracks, even on Germany’s Autobahn, they’re beyond the scope of a consumer publication.

What you do need to know, however, is that you shouldn’t mess around with speed ratings. If your car has a V-rated tire, don’t replace it with an H-rated tire.

Tire Speed Rating Chart

tire speed rating chart tractionlife

M&S vs Mountain and Snowflake symbol

M&S stands for mud and snow, but don’t be fooled into thinking that means such tires are winter tires. They lack the softer rubber and multiple grooves (called sipes) that turn tires from hockey pucks to usable traction devices in winter.

Only tires bearing a symbol showing a snowflake on a mountain are actually winter tires.

When the tires were built is important too

You might also want to know how current your tire is, and there’s a code for that, too. Look for four numbers grouped together. For example, 4617 means the tire was built in the 46th week of 2017.

Other Tire Markings of Note

by Benjamin Yong

Treadwear

Somewhere on the tire you will likely find the word “treadwear” followed by a number: this is rating the lifespan of the tread before it’s replacement time. The benchmark is 100, so a 500 means the tire should have an estimated life of five times longer.

Traction

If you see a letter or letters marked either AA, A, B or C, these stand for how well or poor the tire’s traction is when braking in a straight line on a wet surface. AA is the best, and C is the worst.

Temperature

Either A, B, or C, the temperature rating grades the tire’s ability to withstand heat at high speeds.

Maximum air pressure

This is the maximum pounds-per-square-inch, or psi, of air pressure the tire can be inflated to. Different from the recommended tire pressure found in the vehicle’s user manual or on the driver’s side doorjamb.

Other

M+S, or mud and snow, indicates all-season capabilities and acceptable performance in most weather conditions. May also be represented by icons such as a sun, raincloud, snowflake, etc.

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