Unlike expensive coffee, tires don't come in just three sizes. On the other hand, the tire size branded onto the sidewall gives you more information about a tire's size, purpose, load capacity, and durability than a simple "Tall Decaf" ever will. Once you know what all the numbers and letters mean, you can shop for tires like a pro.
At the end of this article is a complete tire size indicator, which can give you a better idea of how all of this information comes together.
A tire's purpose
Most tire sizes begin with a letter that indicates what a tire was meant to be used for:
P: Tire sizes that begin with a P are the most common. They're designed for passenger vehicles like cars, SUVs, and light-duty trucks.
T: Tires with a T designation are temporary spares — those undersized tires that you aren't meant to use for very long.
LT: LT stands for "Light Truck metric," and these tires are designed for vehicles that haul heavy cargo or do a lot of towing — medium- and heavy-duty trucks and some SUVs and full-size vans use LT tires.
ST: Tire sizes that begin with ST indicate Special Trailer service tires, which are designed for boat, utility, and car trailers and should never be used on vehicles.
No letter: A tire whose size designation doesn't begin with a letter is a metric size used primarily for European cars, but some SUVs and vans might also use them.
You might also find letters at the end of a tire's size designation, which indicate the following:
If it ends in LT: These tires are for vehicles that carry heavy cargo or trailers, that have 16.5-inch rims, or that need oversized tires to help maintain traction on loose dirt or sand.
If it ends in C: These are European tires that are designated for Commercial delivery vans and trucks that carry heavy loads.
A tire's section width
Following the letter is a three-digit number indicating the section width, or the width from the widest part of one sidewall to the other, in millimeters. You can convert millimeters to inches by dividing this number by 25.4.
A tire's aspect ratio
After the section width is a two-digit number that indicates the tire's profile or aspect ratio. It indicates the tire height from tread to rim as a percentage of the tire's section width. See the example at the end of this article to get a better idea of how this works.
A letter after the aspect ratio indicates how the tire is constructed:
R indicates a radial construction, which is by far the most popular. In radials, the tire's plies radiate from the imaginary "center" of the wheel.
D indicates that tire plies criss-cross on a diagonal.
B tires have the same construction as D tires but are reinforced with belts under the treads.
After the internal construction, the next number indicates the diameter of the wheel's rim in either inches or half-inches.
Load index and speed rating
Isolated on the right side of a tire's size designation are its load index and speed rating. The load index is a number that indicates how much weight the tire can hold. Passenger car tires usually have load ratings between 71 and 110, which actually indicate loads between 761 and 2,337 pounds.
After the load index is the speed rating, which lets you know how fast the tires can spin before you risk overheating or blowing the tire. Speed ratings start at M (up to 81 mph) and go up to V (149 mph). Designers then came out with a Z-series tire that was simply rated as "over 149 mph." When they were introduced, Z-series tires were supposed to be the highest speed rating necessary, but as technology improved, the automotive industry backed up and created the W-series (up to 168 mph) and Y-series (186 mph) for exotic sports cars.
Speed ratings were originally established in Europe because of roads there that had no speed limits. Because they were established in Europe, they were rated in kilometers per hour (kph), which is why the ratings in Imperial Units sometimes seem so weird.
Pulling it all together
To understand how all of this comes together, try dissecting this tire size:
Tire's purpose — P: This tire is designed for a passenger car.
Tire's section width — 205: From the widest part of one sidewall to the other, this tire is 205mm, or about 8 inches, across.
Tire's aspect ratio — 70: The tire's sidewall height, from tread to rim, is equal to 70% of the section width. With a 205mm (8-inch) width, the sidewall is 143.5mm (5.65 inches) high.
Internal construction — R: This is a radial tire.
Rim diameter — 16: This tire should be mounted on a 16-inch rim.
- Load index and speed rating — 96T: The 96 translates to 1,565 pounds (710 kg), and the T gives it a speed rating of up to 118 mph (190 kph).