When it comes to changing the tyres on your car, it’s important that you replace what you currently have with tyres that are the same size and are suitable for your vehicle. However, if this is the first time you’ve done this, choosing suitable tyres may seem confusing, especially with so many different types to choose from.
Many car owners aren’t aware of the correct tyre size for their vehicle and it’s common to not be fully clued up on how tyre sizes work. This is to be expected if you’ve tended to leave getting replacement tyres to a professional or if you’ve never had to change your tyres before.
Extra confusion often comes from how tyre sizes are identified, with all of the information being labelled on the tyre in the form of a code made up of a selection of numbers and letters. In this guide, we look at how to check your tyre size and explain what the numbers and letters on your tyres mean.
How do I find my tyre size?
The size of a tyre is printed onto the side of the tyre – on the area known as the sidewall. When looking for the size, you will see that it is made up of a series of numbers and letters.
What do the numbers on tyres mean?
For example if your tyre read '225/55r16 91v' this is what it would mean:
Tyre width (eg 225)
The first three digits. This displays the width of the tyre in millimetres. A tyre marked 225 will measure 225mm across the tread from sidewall to sidewall.
Aspect ratio (eg 55)
The fourth and fifth digits of the tyre code that immediately follow the tyre width. The aspect ratio or profile height of the tyre sidewall is expressed as a percentage of the tyre width. So an aspect ratio of 55 for example means that the profile height of the tyre is 55% of its width.
Radial (eg r )
Radial tyres are marked with the letter R. Radial tyres are constructed with the cord plies positioned at a 90 degree angle to the direction of travel to give the tyre additional strength. Almost every new tyre manufactured today is a radial tyre.
Wheel diameter (eg 16)
The next two digits represent the size of the wheel rim that the tyre can be fitted to. It is also the diameter of the tyre from bead to bead. So a tyre marked 16 will fit on a 16-inch wheel rim.
Load Index (eg 91)
The load index provides information on the maximum weight capability for the tyre. The load index is a numerical code that can be located just after the tyre size marking and before the speed rating.
Speed Rating (eg v)
The speed rating of a tyre is represented by a letter of the alphabet at the end of the tyre size code and indicates the maximum speed capability of the tyre. Tyres receive a speed rating based on a series of tests which measure the tyres capability to handle a set speed for a prolonged period of time.
What does all the other information on my tyre mean?
It’s also possible that a tyre will include other numbers and letters often after these digits for example it may say the following:
XL = Some vehicles require tyres that not only provide exceptional road holding, comfort and noise reduction, but are also capable of carrying significant loads or have stronger sidewalls to deal with power delivery and/or braking - particularly during cornering. These tyres are also designed to take a higher inflation pressure.
C = Commercial Tyre
RFT = Runflat Tyre
MOE = Mercedes Original Equipment key
M+S = Suitable for mud and snow conditions
DOT 36/16 = DOT means that its compliant with department for transport vehicle safety standard. the 36 is the week of manufacture and the 16 is the year of manufacture
SSR = Self-Supporting Run-flat tyre
Can You Have Different Sized Tyres on the Front and Back?
Many cars have the same size tyres on all four wheels, with some manufacturers stating that every tyre should be the same size, brand, speed rating, tread pattern and load index. Due to this, it is strongly recommended that you have the same sized tyres fitted on all four wheels of your car.
However, it may be acceptable to have tyres that are different sizes to each other in some circumstances, but even when this is applicable, it’s only possible if both tyres on the same axle are the same size. For example many Mercedes and BMW's have different size tyres front to rear. If you are unsure of this, ask for advice from a specialist.
If you're ever worried about finding your tyre size or have any questions please feel free to give us a call on 01803 411920