There are alot of questions asked about tyres and what do all the part numbers mean as well understanding what happens to tyre sizes once you increase the size of the alloy and reduce the diameter of the tyre. This short guide will help you to understand the basics.
The example we will use is this tyre as you can see the size stamped on it and this is what you will see on the side wall.
225/35 R 18 W
The first number 225 is the tyre width in millimeters
**The second number 35 (Nominal aspect ratio) is the percentage of the tyre width giving you the height of the tyre.
R = radial construction
The number 18 is the tyre diameter
The W is the speed rating.
When you increase wheel diameters you need to lower the profile of the tyre this is known as up stepping.
When upstepping it is important you retain the rolling diameter of the original factory fitted tyre as specified in the manufactures handbook.
**This is where the aspect ratio is changed so a tyre is taller or shorter even if it is the same width as another tyre eg. 195/50/15 is a taller tyre than a 195/45/15 as the middle numbers are the percentage of the tyre width.
Tyre sizes need to match the overall gearing of the vehicle as this is governed by law.
Speed rating symbols
Finally speed ratings are given here in an easy to read list. All tyres carry a symbol which is a letter to show what is the maximum speed the tyre is to be used at. The list is in MPH.
N = 87 MPH
P = 93
Q = 99
R = 106
S = 112
T = 118
H = 130
V = 149
W = 168
Y = 186
ZR = 149
The current tread depth is that car tyres must have a minimum of 1.6mm of tread in a continuous band in the central 3/4 of the tread width and over the whole circumference of the tyre.
I hope this has helped you to understand the basics of what the different numbers and symbols mean on the side of tyres and the importance of keeping the same rolling radius when upstepping to bigger wheels.If you need anymore help please contact our excellent support staff on the link below.