#TechTip: Tire Sizes 101

Tire sizes can be confusing the first time you have to read them, so here’s a quick run-through:

22 Feb 2016

Tire sizes can be confusing the first time you have to read them, so here’s a quick run-through:

What you'll see on the tire

This is a pretty standard example of what you’d find displayed on a sidewall. There are three measurements on most tires, and they refer to the following:

The width of the tire (in mm)
The sidewall height (displayed as a percentage of the width)
The type of tire and the wheel size

Example:

Here you can see the width is 245mm or a little over 9.5”.
The sidewall is 40% of 245mm wide, putting it at about 98mm on each sidewall
The “R” stands for Radial, and the 18 represents that the tire is for an 18” wheel

Common Misconceptions

“A certain sidewall size is always the same”: Wrong. Remember that the sidewall is a percentage of the width, so a 205/45 will have less sidewall than a 295/45.

“All tire widths are created equal”: Wrong. Different manufacturers have their own biases towards actual measurements. You may find certain brands run consistently narrower or wider than others. Sidewalls should be consistent, however.

What’s your tire size? Post it below!

Comments

Let me add this. If you’re changing the size of your tire, you can use online calculators to check how much the diameter has changed.

225/45/R17, Might go with 235/45/R17 next time because they’re more common and therefore cheaper, and a little wider as well. Catch is I can only fit my finger between the tyre and the front strut as it is lol

Spacers my friend!

Same haha. It’d probably be okay though, only sort of 5mm wider either side of the wheel.

Wish that the 235/40 was as cheap as the 45 profile… Or the 255/40 for meatiness.

225/40/R17 on 8.25inch XXR 527s

255/40/R19 up front and 285/35/R19 for the rears.
There is an android app called Tire Calculator that is useful if you intend to change sizes.
It calculates the total diameter difference and also any speed difference relative to the speedo that has been calibrated to the stock setup.

Another interesting fact:
Tyres will have a date of manufacture on them, it is a 4 digit code, the first 2 digits tell you the week and the last 2 digits tell you the year. For instance this picture shows a tyre manufactured in week 9 of 2007.

In the 80’s it was a 3 digit code.
90’s was a 3 digit code with a triangle after the code.

2000 and onwards was a 4 digit code

And make sure your tires aren’t very old.. there was a documentary showing how tires older than 6 years are dangerous but were still available in some stores

My tire is a truck size, Goodyear PLG8 (also known as “Papaleguas”) Class C 6.50X16,

Fun fact: During the 70’s Michelin tried to reinvent the wheel with TRX by using metric units on the tire instead of imperial. For this to work new wheels had to be built to fit the tires but it flopped as no carmanufactorer backed them up.

Trx & tdx were an early trial at run flat tyres they had a double bead to stop it coming off the rim when driven on flat. Bmw used them on the 5 series Austin also used them in the UK on the metro & montego models. They where a quite expensive tyre & you couldn’t fit any other tyre on the rim