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Tire Sizes: What Do All The Letters And Numbers Mean?

Tire Sizes and What They Mean

Tire sizes can be a bit confusing if you’ve never learned about the different constructions and size formats. Unfortunately, not all tires are created equal, and putting the wrong tire size on your car can create a hazard for you and others around Springfield.

Fortunately, every manufacturer labels their vehicles with the exact kind of tire your vehicle was engineered to use. Whether you’re going to a brick-and-mortar tire shop or using a mobile tire repair service to replace your tires, it’s a good idea to know what you need. Here’s a breakdown of the specifications you’ll find on your tire and what each means.


Tire Sizes: Type of Tire

The first thing you need to know is what kind of tire your vehicle needs. This is the letter that is first in the code you’ll see on your tire sidewall. There are three different types of tires: P or passenger, LT or light truck, and ST or special trailer.

Most personal vehicles use the P-type tire. This covers most cars, wagons, SUVs and smaller trucks.

LT, or light truck, tires are designed primarily for smaller commercial vehicles or trucks with 3/4 or 1-ton load capacity. If you haul a trailer with a substantial load capacity, this would be the kind of tire you would want for your vehicle. Commonly, if you are towing something with a gooseneck or fifth-wheel configuration, you may want to explore LT tires.

ST, or special trailer, tires are those used on most personally owned trailers. This would include your utility, boat, vehicle, fifth-wheel and other travel trailers. If you see ST, keep in mind these are for the trailer, not the tow vehicle.


Tire Width

The three-digit number following your tire type is the tire width. The width of your tire is measured across as you look at your tire head-on while mounted to your car. Width is measured in millimeters, and spans from the outer sidewall to the inner sidewall.

Some people swear by wider tires, but there are some things to know. The idea is that wider tires have more surface are contacting the road. In theory, this increases the traction offered while driving.

However, that is only true in certain circumstances, such as on a racetrack. However, wider tires are also more prone to losing traction on wet surfaces or dirt roads.

Wider tires also impact how your car handles and performs. For instance, wider tires make you turn wider because the tires cannot turn as much. Add to that, wider tires typically increase road noise because of the additional surface contact. If you go too much wider, you may have to change parts of your suspension to prevent other issues or unusual wear.

The best bet is to replace your tires with what was originally equipped on your vehicle. This is the size tire against which the car was safety tested and rated.


Aspect Ratio

After the three-digit width is a slash followed by a two-digit number. Those two digits indicate the aspect ratio of your tire. This is also sometimes referred to as your tire height or profile.

The aspect ratio is a percentage comparing the tire height to the tire width. So 65 would be 65%, or that the tire height is 65% of the tire width in millimeters. If the tire width is 215 mm, then the tire height is 138.75 mm, or 5.5 inches.

The height is measured from the wheel rim to the top of the tire tread when it was manufactured. As the tire wears, this aspect ratio will shrink.


Construction Type

After the aspect ratio is a single letter which identifies the tire construction. The most common construction on modern tires are radial, which is identified with an “R”.

Radial tires have a multi-layer construction. Just below the tread is the cap ply, which is made of parallel rubberized nylon cords that run circumferentially around the tire. This helps protect the rubber tread from the steel cords.

Just below the cap ply are steel cord layers. There are usually two layers of this steel cord, and they run at an angle to the circumferential centerline, creating a crisscross pattern. The steel cord is to help reinforce the tire in the circumferential direction.

Below the steel cord is the radial belts, which run perpendicular to the circumferential centerline. These help provide strength in the radial direction, hence the name radial tires.

Beyond the radial construction, there is also the diagonal construction, indicated by a “D” on the tire. This construction does not have a many layers as radial tires. The interior layer runs in a diagonal pattern, also known as x-ply or cross-ply tires. These are most commonly seen on motorcycle or trailer tires.

Finally, you may see an “F” marking on the construction section of your tire. This indicates the tire is designed as a run flat tire, allowing you to continue driving a short distance if you have a flat to get to a safe place.


Wheel Diameter

After the construction indicator, you will have another two-digit number, which indicates the wheel diameter. It is extremely important you match the wheel diameter both of the wheel to your car and the tire to the wheel. If the wheel diameter doesn’t match from the tire to the wheel, it will not seat properly, if at all.

The wheel diameter is measured from any two points on the wheel where the bead seats into the wheel and runs through the center of the wheel. This measurement is taken in inches.


Load Index

The load index indicates how much weight the wire can sustain to carry. This is typically a two or three-digit number ranging from 65 to 150. 65 indicates 639 lbs capacity, with 150 indicating 7,385 lbs.

It’s important to ensure your load index matches the load capacity of your vehicle. If you go too low, it can create a safety hazard on the roads around Springfield. Overloaded tires are more prone to catastrophic failure.


Speed Rating

Speed rating is the last thing you’ll see on your tire specs, and is typically a single letter. This indicates the fastest safe speed you can travel over an extended period. Driving at speeds greater than the rating risks overheating the tire, losing traction, and experiencing a catastrophic failure. Regardless of the engine potential of your car, your maximum operating speed should never exceed the maximum speed rating of the mounted tires.

MTS Express provides expert mobile tire repair around Springfield. Know that you are getting the right tire size for your car when you work with one of our experienced technicians. Our team comes right to your home or workplace so you don’t have to interrupt your schedule to get new tires.

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