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Car tips & advice | 09.09.2013 - 23:25

How to Tell The Size of Your Car Tires

Being able to understand the number and letters on your tires will help you to identify what sort of tires you should seek when looking for new ones.
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Every vehicle should always be carrying at least one spare tire that matches the ones currently on the road but if you run into an emergency and you have no spare it is good to know exactly what type of tire will get you back on the road. Here are seven steps to figuring out exactly what tires your vehicle is using and which ones might be able to get you that extra five miles down the road to the auto shop.


1.    Reading the Letter(s) on the Tire (P215/60R14 89S)

Most tires begin or end with certain letters. Usually followed by a three digit numeric code, the most common are P, T, LT, C, and ST.

·         If there is a “P” at the beginning, then that tire is a “P-Metric” size that was made primarily for passenger vehicles. This is the most common type of tire as it used on cars, light pickup trucks, minivans, and sport utility vehicles. Sometimes there is no letter at all at the beginning of a tire number. This means the tire is a “Metric” or “Euro-Metric” as they’re sometimes called having originated in Europe. These tires carry the same dimensions as “P-Metric” tires but vary slightly in loading capabilities.

·         If there is a “T” at the beginning of the tire it is a spare, meant only to be used for a short period. Use this tire in the case of emergencies only and use it to drive straight to a shop to get it replaced.

·         If the tire script begins with “LT” then it is a “Light Truck-metric” size. Ironically their description contradicts their purpose a bit as they are meant for carrying heavy cargo, large trailers, basically anything that can be carried without an 18-wheeler. If a tire ends with “LT” they still serve the same basic purpose as those that carry these letters at the beginning except these ones have a wider base are designed to be driven through sand, dirt, anything you might encounter when you’re driving off-road.

·         Some “Euro-Metric” tires end with a “C” and signify that it is a commercial tire intended for use by vans, delivery trucks, and other commercial vehicles. These tires also carry more specific branding for their Service Description. If you are using one of these tires, odds are the company you look for will supply you with new ones.

·         A tire that begins with “ST” is a “Special Trailer Service” tire. Golf cart tires would fall under this category along with those on a boat trailer, wheel barrow, or ATV. You should never use these tires on cars, trucks, or vans.


2.      The Three Digit Number After the Letter (P215/60R14 89S)

This number is the tire’s section width in millimeters. It is the widest point of the inner sidewall to the widest point of the outer side wall when it is mounted on a proper width wheel.


3.      The Two Digit Number After the Section Width (P215/60R14 89S)

This number is the sidewall aspect ratio. Here it indicates that the sidewall height of the tire from rim to tread is 60% of it’s section width. We see that this tire’s section width is 215 mm, so if you want to convert it to inches to figure out the tire section height you would convert this to inches (215/25.4=8.46 inches) and then multiply that number by .60. Therefore we conclude that this tire’s section height is 5.076 inches. This is a very handy number to have in case you need new tires on the go and can’t actually pull the vehicle into the shop.


4.      That Other Letter in the Middle (P215/60R14 89S)

The letter immediately after the section width number identifies the tire’s internal construction. An “R” means that the tire has a radial construction where the plies of the tire’s body radiate out from the center of the wheel. These tires are the most common as they consist of around 98% of tires sold. A “D” instead of the “R” indicates the internal body of the tire plies crisscross on more of a diagonal direction. These tires are typically spares. A “B” indicates that the tire body plies crisscross and is reinforced with belts under the tread area. This “belted” tire is more or less extinct and if you are looking for a new one, odds are your vehicle is on the verge of being so too.


5.      Yet Another Letter in the Middle (P215/60ZR14 89S)

On certain performance tires there is a letter before the internal construction letter. In the past there have been different letters that indicate the vehicle’s minimum top speed. Today the only letter you’re bound to run into at some point is the “Z” which indicates the top speed of the car the tires are on has a top speed of at least 150 mph. These tires are rarely seen outside of the race track and sports cars.


6.      The last 3 Digits (P215/60ZR14 89S)

The last three digits are the service description which represents the tire’s load and speed rating. As stated above, some performance tires carry the speed rating before the internal construction letter but most that carry it at all have it at the end. Different letters represent the minimal top speed rating for the vehicle using those tires and range anywhere from 87-186 mph. The “89” here indicates the load index per tire. An “89” signifies about 1,280 pound per tire.


7.      M+S

The letters “M+S” stand for mud and snow and just serve to tell you that this is an all-season tire. Many of these tires are used by off-road enthusiasts and though they are designed for more hazardous weather conditions, they work just fine on the road after a hard day’s ride through the backwoods.


These readings can all help you and those at the tire shop determine what you need in case there is an emergency. However, it is always better to be safe and have an extra tire that will allow you to resume your trip without having to stop for maintenance. If there is room in your truck try and have a regular tire ready to go rather than a spare. You never know how far away the next mechanic is.


Jack Payton is an auto writer and lifelong grease monkey who loves spending his weeknights at the driving range and weekends in the garage.

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