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Correct tyre size

It is important that you use the correct tyre size for your vehicle to ensure a safe ride.

A knowledge of how the tyre size code works out would come in useful to determine a proper set of tyres and wheels for your vehicle.

The normal tyre size code reads something like 205/45 R17 84W.

The first step is to break it down and define each character.

205 is the section width of the tyre, meaning the width of the tyre patch in contact with the road, measured in millimeters.

Secondly is 45, which is the aspect ratio of the side wall with respect to the section width. This means that the sidewall thickness on this particular tyre is 45 per cent of 205mm.

The R means the tyre is of a radial construction whereas the 17 denotes the diameter of the wheel this tyre will fit, in this case 17 inches.

Finally, the 84 represents the load index, which in this case is 500kg while the W is the speed rating, meaning this tyre can handle speeds of up to 270km/h.

If you are thinking of replacing your original wheels with a size bigger but are afraid of stretching it too far away from the OEM setting, the general rule of thumb is to keep the new overall diameter within three per cent of the OEM size, regardless of whether the difference is less or more.

This is generally referred to as the three per cent rule.

If you are scratching your head to calculate the overall diameter, here is a simple formula and for simplicity sake, we will use the same 205/45/$17 tyre size.

The formula goes as follows : 2(W x AR) + (17 x 25.4).

The W is the section width, AR is the aspect ratio and 25.4 is the conversion factor to convert inches to millimeters. Replace all the necessary numbers and it will read : 2(205 x 0.45) + (17 x 25.4) -- this will give us an overall diameter of 616.3mm.

To go one size up, a suitable tyre size will be 215/40 R18 as this will give an overall diameter of 629.2mm. The difference in diameter is 629.2mm - 616.3mm = 12.9mm.

Once we do the maths, 12.9mm is only 2.09 per cent of the original overall diameter, making this tyre size a suitable upgrade for the vehicle.

If a 225/45 R18 tyre size is used, the difference would be 3.39 per cent, making this size unsuitable for the vehicle.

If you are still confused, consult your tyre dealer.

Remember, using the correct tyre size is important which can prove to be the difference between life and death on the road.


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