If you want to shop smart, it pays to be able to decode this secret language.

If you are in the market for new tires, you might be wondering what the complicated jumble of letters and numbers on the side of your old rubbers mean. Sure, you can simply take your car to the nearest dealership and ask for four new tires, but if you want to shop smart, it pays to be able to decode this secret language.

Let's take an example. Upon closer inspection of a tire's sidewall, you will find something looking like this:

P225/55R18 89H

This tells you pretty much everything you need to know about this tire. Let's go through each component individually.

Learn more about tires:

The P denotes the vehicle class this tire is made for. In this case, it stands for Passenger Car. On pickup trucks or SUV, you might see LT (Light Truck), while your spare probably has a T (Temporary).

The three digits right after are the width of the tire. It is measured in millimetres.

Right after the slash, we find another two digits. Those represent the aspect ratio of the sidewall height. It is a percentage of the total width we previously discussed. If there are no numbers here, it is assumed the percentage is 82 percent.

The letter after our ratio is an indicator of the tire's construction fabric. Most of the time you will only see R (for radial), but on some old units or heavy truck threads, there may be a B (for Bias Belt) or a D (for Diagonal).

Following the single letter are two digits. Those are the load index, and they basically say how much weight each individual tire can support. In this case, our 89 rating means that the tire will handle up to 580 kg.

Lastly, the single letter at the end is your speed rating. This is the maximum speed which those tires shouldn't exceed. Our H here stands for 210 kilometres per hour, which is good enough for most sports sedans. You may commonly see S or T ratings (180 and 190 km/h, respectively), while high-performance vehicle will be shod in expensive W (270 km/h) or even Y (300 km/h) rubbers.

Elsewhere on the sidewall, you might find other information depending on specific applications. There are far too many to list, but let's go through a few important ones:

M+S means it has improved traction in the mud and at lower temperatures. Note that this does not make it a winter tire; those have a snowy mountain pictogram for easy recognition.

If there is an arrow on the side, make sure it is pointing in the direction the tire is rolling; it was designed to perform optimally in one direction.

If you have asymmetric tires, make sure you can spot the letters SFO (Side Facing Outwards); if you see SFI (Side Facing Inwards), your tires have been mounted on the wrong side of the rim!

Photo: greg_bronn on Flickr

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