Can anyone explain tire sizes

[thexav]

What do the numbers mean that are on the side of tires?

Are they all important or can some be ignored?

 

[Lunchmoney]

215/35 R18Z. For example, is my tyre size.
215 is the width of the tyre, looking down. So the amount of tyre on the road. This is measured in mm, millimetres.
35 is the height of the tyre, looking from the side. This is measure as a percentage of the width. Low number = low profile.
R = radius
18 is the radius of the wheel the tyre will fit, in inches.
Z is the speed rating. Mine is quite high.

Some tyres may have M and/or S, which stands for Mud and Snow.

 

[obi_waynne]

The tyre speed ratings, are (in the UK at least)

• N: 87mph
• P: 93mph
• Q: 99mph
• R: 106mph
• S: 112mph
• T: 118mph
• U: 124mph
• H: 130mph
• V: 149mph
• Z: 150+ mph
• W: 168mph
• Y: 186mph

Also you cannot say I don't drive over 70mph so I can stick with the N rated tyre.

The speed rating takes into account the Torque and cornering force with the tyre being approved to reach the stated maximum speed.

If you took an N rated tyre and run it at 150mph it would very quickly shred and split or delaminate.

If you put an N rated Tyre on a 700bhp Skyline the first time you pull a wheel spin you risk shredding the tyre.

Generally speaking you can go with a faster tyre than was approved, but not a slower one.

Winter tyres are also rated to be softer in Winter so although you can use them in summer they will not last very long at all!