How to read a tyre size and load index
Every tyre sold by Tyre Shopper has some markings on the tyre sidewall to indicate key information.
The markings signpost the tyre manufacturer, the tyre size, the tyre speed rating, model and load index. Both low-profile and standard tyres use this marking as standard.
How tyre size works
With a little information, it's easy to understand what the series of letters and numbers mean on your tyre's sidewall.
Here's how to read your tyre markings, taking 165 R 13 79 H for example.
165 – This number is the maximum width of the tyre in millimetres, so here, it's 165mm. This is under conditions when the car is fully loaded.
R – This letter means it is a common radial construction tyre with steel wires. You may find the letter D on vintage cars.
13 – The number is the diameter of the rim, whether metal or alloy. In the example, the tyre is suitable for a 13” diameter wheel.
79 – This is the tyre load index or the maximum capacity that each individual tyre can carry. Here, it's 79 kg.
H – This letter represents the tyre speed rating – or the maximum speed for a given tyre. In this case, it's up to 130mph (209.2kmh).
For your car's tyre pressure, you should always consult your manual.
What does tyre speed rating mean?
Generally, the higher the tyre speed rating, the better quality the tyre. Being aware of your tyres' speed rating allows you to determine the maximum speed your tyre is capable of safely maintaining.
For more information, see our tyre speed rating page.
What is the load index?
Some vehicles carry greater loads and need a higher inflated tyre pressure reading. Tyres designed for higher loads bear the markings XL (Extra Load) or RF (Reinforced).
The tyre load index is the maximum capacity that each individual tyre can carry, usually represented in kilograms.
Tyre sidewall heights
Low profile tyres have a slightly different marking system to regular tyres. After the tyre section width figure, you'll find a number which indicates the sidewall height.
For example, in 185/60 R 14 79 H, the 60 means that the sidewall height is 60% of the tyre section width.
With a standard tyre, the sidewall height is usually 82 percent of the nominal section width.
Can I use a different tyre size?
It's recommended that drivers use the tyre size for a specific wheel size, with a profile and width as noted in your manual.
However, a change in tyre size is possible when bearing in mind the wheel offset / inset value. This avoids over-sized wheels and tyres becoming a hindrance to the functioning of the wheel arch or vehicle suspension.
When changing a tyre size, retain a tyre and wheel assembly diameter of the same value so that the speedometer and gearing remain unchanged.
Basically, for every 10mm boost to tyre width, a reduction of 5% in the height of the sidewall is necessary (if the wheel size is unchanged.)
Selected tyres and wheels must fit under the wheel arch without making contact with it or disturbing the vehicle's suspension.
Our expert guidance for tyre sizes
Tyre Shopper can provide expert advice to buyers looking for tyres online, whether to change their tyre size or wheel size, so contact us today. We have 230 fitting centres across the country.
Please note that such changes are considered a modification by some UK insurance companies.
Back to Tyre Information
You may also be interested in the following article:
Tyre Sizes Explained provides you with further information to help you understand which tyre size you will need for your vehicle.