Tyres 101

Tyres are more than just black and round... They utilize an innovative mix of chemistry, physics and engineering. And we're here to help explain. Learn how to read your sidewall, understand tyre construction and speed ratings, and find the right tyres for your car.

Reading a tyre side wall

The numbers and letters on the side of your tyres have very specific meanings.

Tyre Brand

Tyre brand

This is the brand or manufacturer of your tyre.

Tyre Pattern Name

Tyre patern name

The tyre pattern name is the model or name designated to a particular tyre – this information is usually found after the manufacturer’s name on the sidewall.

Tyre Type

This designates the type of vehicle the tyre fits. P is for passenger metric. Other letters are LT (for light truck), T (for temporary spare) and ST (for special trailers). If your tyre has no letter, it signifies that your tyre is a euro “metric” size.

Tyre type

Tyre Width

The width of your tyre from sidewall to sidewall. In this example the width of the tyre is 225mm.

Tyre width

Tyre Aspect Ratio

This identifies the tyre’s aspect ratio, which is the relationship of the tyre’s sidewall height to the tyre’s width. In this example, the sidewall height of the tyre is 55% of its width. The lower the ratio, the smaller the sidewall height, which means better cornering, but a rougher ride.

Tyre aspect ratio

Tyre Construction

This is the tyre’s internal construction, which is “radial.” Almost every tyre on the road has radial construction, which means the cords of the carcass plies inside the tyre “radiate” directly across from one side of the tyre to the other. Other letters used are D, for diagonal construction, and B, for belted.

Tyre construction

Wheel Diameter

This number (in inches) indicates that the tyre is designed to fit on a wheel with a 18-inch diameter.

Tyre Wheel parameter

Load Index

This indicates how much weight the tyre is certified to carry at maximum safe inflation. Normally tyres can carry between 60 (250 kg/tyre) to 110 (1060 kg/tyre).

Load index

Speed Rating

This indicates the maximum safe speed at which a tyre is certified to travel under specified conditions. Speed ratings range from A (the lowest) to Y (the highest).

Speed rating

Tyre Speed Ratings & Tread Life

The most common tyre speed ratings, speeds and vehicle usage are as follows:

Tyre speed rating
The right tyre for your car

Your correct tyre size and tyre pressure can be found in your vehicle’s owners’ manual in the glove compartment or on the OEM sticker on your driver’s side door.

Tyre Size

OEM stands for Original Equipment Manufacturer. Quite simply, these are the tyre specs that originally came with your vehicle, according to the manufacturer’s specifications and requirements. It includes the tyre size, rim size, the aspect ratio etc.

Placard

The Origin of Speed Ratings

We can thank Germany’s famous Autobahn for tyre speed ratings. Tyre speed ratings range from A (the lowest) to Y (the highest). But the chart is not completely in alphabetical order. For example, H is between U and V, with the common perception that H stood for “high performance” at one time. As manufacturers continue to add speed to their vehicles, tyre speed ratings evolve to match the speeds. For example, Z was the highest rated speed at 149+ until W & Y were used to match the higher speeds of exotic sports cars.

Speed Ratings Refer to More Than Just Speed

Speed ratings make a difference not only in regards to speed, but in regards to ride comfort, wear and cornering ability. Typically, the higher the speed rating, the better the grip and stopping power, but the lower the tread life. You can always increase the speed rating of the tyres on your vehicle for improved performance, but can never decrease it without reducing the vehicle top speed to that of the lower speed rating selected.

Mixing Speed Ratings

If tyres of different speed ratings are mounted on a vehicle, the lower speed-rated tyres should be placed on the front axle regardless of which axle is driven. This is to prevent a potential oversteer condition. Vehicle handling may be affected, and the vehicle’s speed capacity is now limited to the lowest speed-rated tyre. For best performance, it is recommended that the same size and type of tyre be used on all four wheel positions.

Load Index

“235/55R17 99H- The load index (99) is the tyre size’s assigned numerical value used to compare relative load carrying capabilities. The higher the tyre’s load index number, the greater its load carrying capacity.

  • 97 = 1,609 pounds/729 kilograms
  • 98 = 1,653 pounds/749 kilograms
  • 99 = 1,709 pounds/775 kilograms

A tyre with a higher load index than that of the Original Equipment tyre indicates an increase in load capacity. A tyre with a load index equal to that of the Original Equipment tyre indicates an equivalent load capacity. A tyre with a lower load index than the Original Equipment tyre indicates the tyre does not equal the load capacity of the original and should not be considered for installation on the vehicle.

Typically, the load indexes of the tyres used on passenger cars and light trucks range from 70 to 130.

Speed rating load index