Tyre Sizing

Tyre Sizing can be quite confusing without correct references - so below is a basic toolkit to allow you to understand the way tyre sizing works.

There are three types of tyre sizing throughout the work. Currently the most common is Metric Tyre Sizing. This is shown below. Metric Tyre sizing is used on almost all passenger vehicles, SUVs, 4WDs, Pickup Trucks, Performance Cars, and Muscle Cars on the Road Today. Some Trucks also use Metric Tyre Sizing, however most new trucks now come with Metric Tyre Sizes and most manufacturers are now changing over to Metric Tyre Sizes.

Metric Tyre Sizing Example

Metric Tyre Size

Metric Deconstruction

   

Metric Tyre Sizing Defined

The above example shows a 235/45R17 in this the tyre size is broken down as follows:
- The "235" shows that the tyre is 235mm wide (Shown above in Red). The width is the part that has the Tread Pattern on it.
- The "45" shows the tyre profile is 45% of the width (235 multiplied by 0.45 equals 105.75mm) therefore the 235/45R17 has a sidewall height of 105.75mm.
- The "17" is simply the rim size, this is the only part of the tyre size which is measured in inches, so the tyre must be mounted on a 17 Inch Wheel
 

Common Tyre Size Letters

Description

"ZR" instead of "R" This still means that the tyre is of Radial Ply Construction, the only difference is that its speed rating (See more on Speed Ratings Below and in the Speed Ratings Tab under tyre information), is in excess of 240Klms/H.
"VR" Refers to the same construction as "ZR", but the proper speed rating is listed after the tyre size.
"SR" instead of "R" SR stands means the same as R, the tyre is of Steel Radial Construction.
"C" after Rim Size "C" stands for "Commercial" it means there is extra radial belts within the construction of the tyre and/or sidewalls.
"LT" after Rim Size "LT" Stands for "Light Truck" it is usually a step up from the commercial rating in most cases, but has similar load and speed ratings.
   

Imperial Tyre Sizing Example

Imperial Tyre Size Imperial Tyre Size Deconstruction
Common Tyre Size Letters Description
"C" as the last digit "C" stands for "Commercial" it means there is extra radial belts within the construction of the tyre and/or sidewalls.
"LT" as the last digits "LT" Stands for "Light Truck" it is usually a step up from the commercial rating in most cases, but has similar load and speed ratings.

Truck Tyre Sizing Example

Imperial Tyre Size Imperial Tyre Size Deconstruction
Common Tyre Size Letters Description
"C" as the last digit "C" stands for "Commercial" it means there is extra radial belts within the construction of the tyre and/or sidewalls.
"LT" as the last digits "LT" Stands for "Light Truck" it is usually a step up from the commercial rating in most cases, but has similar load and speed ratings.

What is an Equivalent Tyre Size

An Equivalent size is a tyre size which can be slightly wider, narrower, but keeps the same rolling circumference as the original tyre therfore making is completely legal and within manufacturer specifications. Equivalent sizing is common to raise Raise the level of grip, or cornering ability while reducing initial tyre costs and gaining access to a far larger number of tyre options. A standard alternate tyre size which is commonly used is to increase the width by 30mm while subtracting 5% from the tyre profile. This is demonstrated below.

The extra width increases contact area with the road allowing for more grip when cornering and better stopping distances when braking. As the sidewall profile is a percentage, the 245/35R20 shown below works out to be the same overall height as the 275/30R20 when the profile is reduced by 5%. This is a perfect example of Equivalent Sizing.

 

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