How to Read Car Tire Sizes & Tire Sidewall Markings

If you're buying a new set of tires, it's essential to know what those sidewall markings mean - fortunately, it's not difficult to understand. We break it down, including tire load index and speed ratings (both with charts).

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.

In this article, we discuss:

  1. What tire sizes mean and how to read them
  2. The tire load index
  3. Speed ratings explained

1. What do tire sidewall markings mean, and how do you read them?

Here’s a standard tire size: P215/60R16 – as shown below:

Photo: Goodyear
  • 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 215 is the tire’s width in millimeters from sidewall to sidewall.
  • The 60 refers to the ratio between the width and height of the sidewall. In this case, the sidewall is 60 percent of the width, 60 percent of 215.
  • The R means radial, as in the construction style, while the 16 means it rides on wheels with 16-inch diameters.
  • 1. Metric, or tire type
  • 2. Tire width
  • 3. Rim diameter
  • 4. Speed rating
  • 5. Aspect ratio
  • 6. Construction
  • 7. Load index

When the tires were built: here’s how to know

You might also want to know how current your tire is; there’s a code for that, too.

Look for four numbers grouped. For example, 0918 in this picture means the tire was built on the 9th week of 2018.

Reading a tire size correctly is essential, and here’s why

These numbers are necessary 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 percent aspect ratio (sidewalls are only 91.5 mm high), on 20-inch wheels.

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

tire sidewall markings 3 types offered by goodyear
  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

2. What is the tire load index?

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

For example, the tires with a rating of P225/65R17 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

Please note that a speed rating symbol typically follows the load index, and both are marked on the tire’s sidewall to indicate its load and speed capabilities. Always ensure that the load index of your tires meets or exceeds the requirements for your vehicle and intended use.

Load IndexMaximum Load Capacity (lbs)Maximum Load Capacity (kg)
71761345
72783355
73805365
74827375
75853387
76882400
77908412
78937425
79963437
80992450
811019462
821047475
831074487
841102500
851135515
861168530
871201545
881235560
891279580
901323600
911356615
921389630
931433650
941477670
951521690
961565710
971609730
981653750
991709775
1001764800
1011819825
1021874850
1031929875
1041984900
1052039925
1062094950
1072149975
10822051000
10922711030
11023371060
11124031090
11224691120
11325351150
11426011180
11526791215
11627561250
11728331285
11829101320
11929981360
12030861400

3. Speed Tire Rating Explained

Tires are under extreme amounts of force, not only in holding up the vehicle above but at speed. There are also excessive amounts of centripetal and centrifugal force acting on the tires. The centripetal force refers to the forces acting in line 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 of 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 by 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, which can bring the car to a halt at such high speeds. Since these higher speeds are typically only seen on racetracks, even on Germany’s Autobahn, they’re beyond the scope of our discussion here.

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

Note: it’s essential to choose tires with a speed rating that matches or exceeds your vehicle’s top speed capabilities and driving conditions.

Speed RatingMaximum Speed (km/h)Maximum Speed (mph)
L12075
M13081
N14087
P15093
Q16099
R170106
S180112
T190118
U200124
H210130
V240149
W270168
Y300186
(Y)Over 300Over 186

There you have it, an in-depth look at that letter and digits on passenger vehicle tire sidewalls and what they all mean. At first glance, it’s all quite overwhelming. But once you break it down into each part – from tire size and load index to speed rating and symbols – it begins to make sense.

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Author:
Kelly Taylor
Kelly Taylor
Kelly Taylor has been writing about cars since 2000. His favourite ride has been the Audi R8 from Thunder Bay to Winnipeg, where he nearly traded the car for a Ford Ranger, a Greyhound Bus and the Blue Heron Gift Store in Kenora, Ont.