Tire Sizes Explained

Modern automobiles come in varying sizes from the small cars to the large trailers. They all come fitted with tires proportionate in size. None of them would move an inch from their place without the tires which are the most essential car parts! Every car, truck, van or trailer needs the right size of tire with the right specifications that can effectively carry its load.

Automobile manufacturers have their own 'secret' tire code, that details all the information about every tire. This coded information is usually etched on the side of every tire. It is an alphanumeric code which you need to crack in order to know which kind of automobile, the tire is meant for, what is its size, width, aspect ratio, load handling capacity and speed rating. This article is the 'Rosetta Stone' of the tire code that you need, in order to decipher what the series of alphabets and numbers etched on tires means!

Code For Tire Sizes

There are many factors that determine a tire size which includes the tire's utility purpose, section width, aspect ratio, internal construction, rim diameter, speed rating and load capacity. To indicate all these factors related to a tire size, an alphanumeric tire code was devised that would do the job. Just a glance through the code can tell you what you need to know about that 'tire size'. Here are the various terms in the code, deciphered for you. (The factors appear in the order according to which they appear in the tire code.)

Tire Utility
Let me explain tire sizes, starting with the first letter of the code, etched on the tire side. The most important tire rating is the utility part (denoted by single or double alphabets) which indicates what kind of vehicle that tire is meant for. Here is the code unraveled for you.

• P: Meant for passenger vehicles including cars, light-duty trucks and sports utility vehicles
• T: These are spare tires used as temporary substitutes
• ST: Meant for special trailer vehicles like car trailers and boat trailers
• LT: Meant for light trucks, medium and heavy duty trucks
• No Letters: Indicates a tire meant for European cars, vans and SUVs
• Ending With LT: These are meant for European heavy cargo or trailer vehicles
• Ending With C: This is the European designation for tires fitted in commercial vehicles like trucks and vans

Tire Section Width
After the alphabet designating the utility purpose of the tire, you will find a three digit number that is the section width of the tire. Section width is the distance between most distant parts of the tire's two side walls. It is always mentioned in millimeters.

Tire Aspect Ratio
Following the section width measurement, you will have a two digit number that indicates the tire aspect ratio. This is the ratio of rim to tread width with the total tire section width expressed in percentage.

Interior Construction
Next in line is another alphabetical one letter code, that indicates interior construction of the tire. It may be 'R', signifying radial architecture, 'D' indicates a diagonally crisscrossing architecture, 'B' signifies crisscrossing diagonal architecture reinforced with belts.

Diameter of Tire Rim
In tire size explanation, the next in line after the type of interior construction, is a numerical value of the tire's rim diameter in either half inches or inches.

Load Index
Separately, on the right hand side of tire size details, the load index and speed rating are indicated. Load index is the maximum load that the tire can safely handle. Load rating is a number between 71 and 110, that corresponds to a load range between 761 and 2337 pounds.

Speed Rating
Another specification indicated separately is the speed rating of a tire, that indicates the maximum speed at which the tires may spin safely. This is indicated by a single alphabet. The ratings indication is deciphered as follows:

• M: Maximum Speed Up till 81 miles per hour
• V: Maximum Speed Up till 149 miles per hour
• Z: Over 149 miles per hour
• W: Maximum Speed Up till 168 miles per hour
• Y: Maximum Speed Up till 186 miles per hour

Sample Code Example

Let us apply the tire code information to interpret what the tire size specification P215/70R16 100V means. 'P' indicates that this is a tire intended for a passenger vehicle, 215 mm is the section width, aspect ratio is 70% of section width, interior construction is of radial type, rim diameter is 16 inches, 100 is the load rating corresponding to 1764 pounds and V indicates a maximum speed rating of 149 miles per hour.

A tire size comparison chart found in car spare part retail stores will list all these specifications in more detail. How and when to replace tires is your prerogative, but make sure that you check the tire ratings beforehand.

The code for different tire sizes shared here, along with specifications will enable you to shop for your car's tires on your own. Read the code on the side of your old tires and note it down. You will require the same tire model with the same code etched on the side. Hope this 'Rosetta Stone' has helped you out.