The SpeedGirls Magazine


Cracking the Tire Code

Deciphering the Numbers on Sidewalls
Tire Size Codes


GAH! Does this number look like a algebra equation? Well, it’s actually data to determine the size specifications on a tire, and if the numbers leave you a bit cross-eyed — it’s OK. Because with simplicity, these numbers can be broken down like a code, and in no time you’ll know exactly how to decipher the right tire size for your vehicle.

Starting with the first three digits (295/), which represent the tread width.  This number measured in millimeters from sidewall to sidewall.  So just cheat and use a converter to break it down into inches.  Overall a larger number will give you’ll more tread contacting the ground surface, but don’t think this will always make your tire wider.  Keep reading.

The second number is aspect ratio (/30R), which represents sidewall height.  This number is the percentage of sidewall height versus the tread width.  So 30 means, your sidewall is 30% of the tread width.  Example: Your tread is 100 millimeters wide. The sidewall is 30 millimeters tall. Your side wall would be 30% of the tread width.  The “R” just signifies radial, meaning the constructed layers in the tire run radially across the tire.

Tread width and aspect ratio can dramatically affect how your tire looks on the wheel.  A large tread width combined with a large aspect ratio can give you a mushroom effect.  Where as the opposite, a smaller tread width and smaller sidewall will make your tires look like they where stretched onto the wheel.  Most wheel manufactures will provide the optimal tire size to create a balanced look for you.

The third series (19-) is the wheel height.  This is measured in diameter across the tire opening.   Not the height of the wheel.  The number 19 represents a 19-inch wheel size.

The final number series (99) is the load index.  This number is a cross reference to indicate how much weight the tires can handle.  Of course, this really isn’t a major a factor performance cars, but for trucks or light duty vehicles.  It’s very important.

Lastly, we have our most favorite and most costly factor.  The speed rating (Z).  Think about this letter as an alphabetical race license.  The higher of letter, the faster the tire is rated to drive.  Being “Y” is the highest at 186mph.  The letter “A” would be for a Prius wagon.