How to Read Tire Measurements

Tire Tread

Deciphering the long string of letters and numbers on the sidewall of a car tire may be confusing at first glance, but they provide important information about the tire's dimensions, load capacity, intended purpose, internal construction, and durability at high speeds. This article describes how to read those puzzling tire measurements.

Difficulty:

Moderately Easy

Instructions

Things You'll Need

• Car tire
• This article, printed out for your reference

1. These instructions follow the example of a P195-70-R14 90T tire.

2. Service Type

   The majority of tire sizes start with a letter that identifies the type of vehicle and/or type of service for which they were designed:

   P = P-metric or Passenger. The most common service type, this signifies that the tire is fitted on passenger vehicles: minivans, cars, SUVs, and light trucks.

   T = Temporary Spare. Yep, it's the diminutive spare tire hanging out in your trunk.

   LT = Light Truck-metric. This signifies that the tire is fitted on trucks that are capable of towing trailers or carrying heavy cargo: full-size vans, large SUVs, and pickup trucks.

   ST = Special Trailer Service. This tire is only used on boat, utility, or car trailers. They are not designed to be fitted on motor vehicles.

3. Section Width

   The three numbers after the first letter signify the tire's width in millimeters. For the P195-70-R14 90T tire, that's 195 millimeters if you're measuring across widthwise from the outer sidewall to the inner sidewall.

4. Sidewall Aspect Ratio

   The two numbers after the section width identify the tire's profile (aspect ratio). For the P195-70-R14 90T tire, that would be 70.

   What's an aspect ratio, you ask? Here's where it gets technical: 70 indicates that the sidewall height (from rim to tread) is 70 percent of its section width.

   What does this equation tell us? Higher numbers mean taller sidewalls, and lower numbers mean shorter sidewalls.

5. Internal Construction

   The letter after the two numbers representing the sidewall aspect ratio signifies the internal construction of the tire.

   Per the P195-70-R14 90T tire, the R means radial, the most common tire construction. However, a D would mean diagonal construction which is used mainly for applications in spare tires and light trucks.

6. Tire and Wheel Diameter

   The two numbers after the letter identifying the internal construction indicate the diameter of the wheel in inches.

   For the P195-70-R14 90T tire, the tire and wheel diameter is 14 inches. However, many new compact and even subcompact cars today are moving up to the more luxurious 15-inch tires.

7. Service Description

   The last set of numbers followed by a letter indicate the tire's load carrying capacity index (maximum weight on the tires) and speed rating (how fast the tires are designed to go).

   Using the example of the P195-70-R14 90T tire, that is 90T:

   90 = 1323 pounds max (600 kilograms)
   T = 118 MPH max (190 km/h) for passenger cars

Tips & Warnings

• Keeping your tires properly inflated extends their lifetime and increases fuel efficiency.

• Many newer cars are equipped with a tire pressure monitoring system (TPMS) that notifies the driver of any decrease in tire pressure.

Resources

• Tire Rack.com: A Guide to Tire Sizes