Understanding Tire Sizes

Written by Chris Dekker, former co-owner at Tools in Motion.


We get a lot of questions about tires, and tire sizing, which is no surprise because the sizing format that has become the industry standard couldn’t be more confusing. What do all those numbers on the sidewall of your tire mean? Let’s break it down:


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  • The section width is pretty straightforward. This is the width of the tire, in millimeters.
  • The aspect ratio is where things start to get confusing. This is the height of the tire sidewall (from rim to tread), but this is expressed as a percentage of the width. For example, this tire’s sidewall height is 75% of 185mm, which is a little under 140mm. Because the aspect ratio is a percentage, a 195/75R14 tire is actually wider and taller than say, a 185/75R14 tire.
  • The “R” construction method means this is a radial tire, as with almost every tire produced today.
  • The rim diameter is expressed in inches – yes, we’re mixing metric and imperial measurements here for some reason! This tells us what diameter of rim this tire will fit on.

These measurements all often all that is considered when most customers shop for tires, but there are two more very important numbers on the side of your tires that shouldn’t be overlooked:

  • The load rating tells us how much weight the tire can support at its maximum air pressure without failing. For obvious reasons, your tire’s load rating should meet or exceed what is required by your vehicle.
  • The speed rating of the tire is often misunderstood. In theory, this number tells us how fast a tire can be driven without failing, but even cheap “S” rated tires are rated for 180 kilometers per hour! The ratings continue to increase as you move towards the end of the alphabet.

So, why not just put “S” rated tires on every vehicle? You’re not going to drive that fast, right? In reality, the speed rating tells us a lot more about the construction and stiffness of the tire – how it brakes, corners and grips – even how much it heats up travelling down the road. It is absolutely essential that your tire speed rating meets the standard required by your vehicle in order for it to handle and perform the way the manufacturer intended.

Recently, Global News showed a story where they equipped two Mazda sedans with new tires; one with the recommended “V” rated tires, and one with cheaper “S” rated tires. Not only did the car with the right tires ride and handle better, it also stopped a whopping 23 feet sooner when both vehicles had to brake hard from 80 km/h. This could be the difference between life and death in some situations.


Airdrie Tire Shop


Most newer vehicles have a tire size placard, similar to the one above, which will show you what size (plus load and speed rating) of tires your vehicle requires. It will also show you the manufacturer’s recommended tire pressure. A quick note on this: Always set your tire pressures to the recommendation on this decal. The tire pressure shown on the sidewall is the theoretical “maximum pressure” of the tire, and may be way too high for your vehicle!

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