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Tyre Information: Understanding and Reading a Tyre Size and Load Index

Every tyre sold by Tyre Shopper has some markings on the tyre sidewall to indicate key information about that particular tyre product.

How do I read the markings on the sidewall for my tyre speed rating?

The markings indicate the tyre manufacturer, the tyre size, the tyre speed rating, its model, and other information. Both low-profile and standard tyres use this marking standard.

Here is an example:

165 R 13 79 H

165 – Refers to tyre section width of 165 millimetres

R – Radial tyre type

13 – Tyre suitable for a 13” diameter wheel

79 – Tyre load index

H – Tyre speed rating of up to 130mph (209.2kmh)

With tyre speed ratings, there are several levels:

S = Up to 113mph

T = Up to 118mph

H = Up to 130mph

V = Up to 149mph

Z = More than 150mph

W = Up to 168mph

Y = Up to 186mph

It is a good idea to only replace a worn tyre with one that has the same or a greater speed rating. As a general rule, a higher speed rating means that the tyre is a better quality product. The improved quality is reflected in superior cornering power, better braking, improve grip to wet roads, and a more responsive steering.

Load Index

Depending on the car, there are some models with a requirement to carry greater loads and those need a higher inflated pressure reading. Check the manufacturer’s manual or handbook to confirm this information. Tyres that are designed for higher loads bear the markings XL (Extra Load) or RF (Reinforced).

Low profile tyres have a different marking system to regular tyres:

Here is an example:

185/60 R 14 79 H

185 - Refers to tyre section width of 185 millimetres

60 – Indicating that the sidewall height is 60% of the tyre section width

R – Radial tyre type

14 - Tyre suitable for a 14” diameter wheel

79 = Tyre load index

H - Tyre speed rating of up to 130mph (209.2kmh)

With a standard tyre, the sidewall height is usually 82 percent of the nominal section width. When cutting the percentage of sidewall height compared to tread nominal section width (known as the "aspect ratio"), this enables wider tyres to be used that have extra tread to help grip the road better.

Is it possible to use larger tyres?

Whilst it is recommended that the driver use the tyre size for a specific wheel size, with a profile and width as noted in the owner’s manual, a change in size is possible when bearing in mind the wheel offset / inset value. This avoids over-sized wheels and tyres becoming a hindrance to the proper functioning of the wheel arch or suspension on the vehicle.

When making a change in tyre size, be sure to retain a tyre and wheel assembly diameter (aka its rolling circumference) of roughly the same value so that the speedometer and gearing remain unchanged. For more information on this point, take a look at this Car Bibles article which also includes a useful calculator to experiment with different size combinations.

A basic rule to use is that for every 10mm boost to tyre width, a reduction of 5% in the height of the sidewall (known as aspect ratio) is necessary. This calculation only makes sense if the wheel size is unchanged.

For illustrative purposes, it is possible for a 165/70 R13 T tyre to be switched for a 175/65 R13T tyre. Using the calculator from the linked Car Bibles website, the rolling circumference changes with the tyre switch from 1692.54mm to 1681.98mm which is only a (0.62%) or (10.56mm) difference. Aiming for a maximum 3 percent variance when swapping wheels and tyres is a good rule of thumb here.

With that said, the tyres and wheels selected must fit under the wheel arch without making contact with it or being a problem for the vehicle’s suspension.

Tyre Shopper Provides Expert Guidance for Tyre Size Changes

Our staff at Tyre Shopper can provide advice to tyre buyers looking to change their tyre size and/or wheel size for something different. Staff can run through the options available with the customer’s vehicle. Call us or email at customerservices@tyreshopper.co.uk.

Please bear in mind that a tyre size / wheel side change is considered a “modification” by some insurance companies in the UK. The insurer needs to be contacted, before making a purchase, to verify that they will continue to insure the vehicle once upgraded.

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