How to Determine the Tire Size of Your Vehicle

Understanding Tire Sizing Systems

There are two central tire sizing systems, both of which are measured in millimeters (mm).

P-Metric Sizing

The P-metric sizing system applies to most passenger cars, station wagons, SUVs, and some light-duty pickup trucks. They are easily identifiable with the “P” at the start of their code. The P-metric sizing system is typically used in passenger cars. The P-metric system calculates tire load capacity by considering a tire’s physical size and air pressure.

P-Metric tire types are as follows:

  • P (Passenger) - Tires designed for cars, minivans, and SUVs. They offer a balance of ride comfort, handling, and fuel efficiency.
  • LT (Light Truck) - These tires are designed for light trucks, SUVs, and vans and feature enhanced load-carrying capacity and durability. They are ideal for heavier vehicles and towing.
  • ST (Special Trailer) - For special use in trailers. ST tires have stiffer sidewalls designed to handle vertical loads.
Metric and Euro-Metric

Metric or Euro-metric size tires do not have the “P” at the beginning. They are also primarily used on passenger vehicles but differ in load capacity calculations and inflation tables. Euro-metric tires typically have a slightly higher indicated load index reserve than P-metric tires.

Tires from both systems are considered interchangeable and equivalent provided:

  1. They have the same numeric size, performance category, and speed rating.
  2. They are used either in axle pairs (on the same axle) or sets of four.
  3. The inflation pressure provided in your owner’s manual is followed.
  4. Your vehicle came with P-metric tires. The inverse is not applicable because P-metric tires have a lower indicated load index reserve.
Finding the Original Equipment Tire Size

Decoding Tire Size Numbers

The code printed on your tire’s sidewall represents your tire’s dimensions and rim compatibility. The size code takes either of these formats: P225/55R17 or 225/55R17. A breakdown of each character is as follows:

Prefix / Tire Type (P)

Defines the type of vehicle the tire is intended for (P, LT, or ST). The example tire is designed for passenger vehicles. The “P” also indicates that the tire measurements are based on P-Metric sizing.

Tire Width (225)

It represents the measurement (in millimeters) from sidewall to sidewall. The example tire has a width of 225mm.

Aspect Ratio (55)

The ratio of a tire’s height to its width is represented as a percentage. A percentage is used because it measures the area spanning the sidewall’s height (from alloy rim contact to outer tread) or the sidewall’s “profile.” This means the example tire’s section height is 55% of its section width (123.75mm).

Construction Type ®

Represents a tire’s internal construction (whether radial or cross-ply). A tire’s internal construction can affect riding characteristics. Radial tires are designed for comfort, water and heat resistance, and fuel economy. Cross-ply tires have higher rolling resistance and are more resistant to sidewall damage. The example tire has radial internal construction.

Rim Diameter (17)

Measures the diameter of the tire’s inner rim (in inches) and indicates the wheel size your tire is intended for. The example tire is designed for use with a 17-inch wheel.

Therefore, the tire code “P225/55R17” means the following:

  • The tire is rated for use on a passenger vehicle.
  • The tire has a width of 225mm.
  • It has a section height of 123.75mm.
  • The tire Has a radial internal construction.
  • It is intended for use with a 17-inch wheel.

Service Descriptions

A complete tire code looks like this: P225/55R17 97V. The last two designations do not relate to tire size but instead represent the service descriptions for which the tire is designed.

Load Index (97)

The tire's load index is the two- or three-digit number following the gap. The number corresponds to a load capacity (in lbs.) listed in a load index chart. The example tire has a load index of 97, which translates to a max capacity of 1609 lbs based on the load index chart.

Speed Rating (V)

Represents the top sustained speed you can safely travel at. A tire with a high-speed rating can handle the heat and provide control at higher speeds. The example tire has a V speed rating, which translates to a top operating speed of 149 mph based on the speed rating chart.

Tire Identification Number

The tire identification number starts with a DOT symbol followed by eight to 13 characters representing DOT information. The DOT symbol signifies that the tire complies with the U.S. Department of Transportation’s safety standards. The DOT information identifies:

  • Manufacturing location
  • Tire size code
  • Tire type code
  • Week of manufacture (two numbers)
  • Year of manufacture (two digits)
Special Tire Considerations

Tires also come in unique treads that tackle varying road and climate conditions, such as all-season, summer, or winter tires. Some tires are also specially designed for performance, with enhanced traction, handling, roll, and heat resistance.

Consider the intended use and climate in selecting the appropriate tire and tread pattern.

Significance of Manufacturer’s Recommended Sizing

While a larger tire size can also translate to improved traction and higher ground clearance, upsizing is discouraged because it affects the key characteristics of your vehicle. Manufacturer-recommended sizing is designed to yield optimal vehicle performance.

By deviating from recommended values, car owners alter driving characteristics:

  • Decreased acceleration and fuel economy - Larger wheels require greater engine power to rotate. Larger tires also add to the vehicle’s weight and create drag.
  • Inaccurate speedometer reading - Speedometers are finely tuned to the combined diameter of the wheel and tire. As such, those increasing wheel sizes must also decrease the tire aspect ratio to maintain speedometer accuracy. This also entails using tires with stronger sidewalls to withstand the lateral forces in play during cornering.
  • Decreased ride comfort - A smaller aspect ratio means there’s less room for flex for wheels to absorb shock on roads.
  • Larger tires translate to heavier steering - While wider tires improve traction and stability, they require tremendous effort to steer and control. This can often result in understeer, which presents a significant disadvantage in slow traffic.

Manufacturer Recommendations and Warranty Coverage

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