How to Measure Mounted Tire Width and Height

To measure a mounted tire's width and height, you can simply read the size markings on the sidewall, which give the tire's section width in millimeters, aspect ratio, and rim diameter in inches. The actual overall width and height can also be physically measured with a tape measure: overall width is the distance between the outer edges of the sidewalls at the widest point (excluding raised lettering or ribs), and overall height (diameter) is the distance from the ground to the top of the tire when inflated and mounted under the vehicle's load, or the maximum diameter when off the vehicle and inflated.

Understanding Tire Sidewall Markings

Tire sidewalls contain a series of numbers and letters that indicate the tire's size, construction type, and performance capabilities. For example, a common tire size is P225/50R17 93V. Here's what each part means:

• P – Tire type: P stands for P-Metric (passenger car). Other prefixes include LT for Light Truck, T for Temporary spare, and ST for Special Trailer.
• 225 – Section width in millimeters: the width of the tire from sidewall to sidewall when mounted on a specified rim width and inflated to the recommended pressure. This is not the tread width.
• 50 – Aspect ratio: the ratio of the tire's sidewall height to its section width, expressed as a percentage. In this example, the sidewall height is 50% of 225 mm, which is 112.5 mm.
• R – Construction type: R stands for radial construction, which is used in almost all modern tires.
• 17 – Rim diameter in inches: the diameter of the wheel (rim) that the tire is designed to fit, measured from bead seat to bead seat.
• 93 – Load index: a numerical code that corresponds to the maximum load the tire can carry at the speed indicated by its speed rating.
• V – Speed rating: a letter that indicates the maximum speed at which the tire can carry its maximum load.

So, from the sidewall markings, you can determine the tire's nominal section width (225 mm), the sidewall height (112.5 mm if the aspect ratio is 50), and the rim diameter (17 inches). To get the overall diameter, you add twice the sidewall height to the rim diameter (after converting units appropriately).

How to Measure Tire Section Width

The section width is the linear distance between the outer sides of the sidewalls of an inflated tire, excluding any raised letters, ribs, or protective ribs. It is measured at the tire's widest point, usually near the middle of the sidewall, when the tire is mounted on its measuring rim and inflated to the specified pressure. The measuring rim width is the industry-standard rim width on which the tire's dimensions are determined; for a 225 mm tire, the measuring rim width might be 7.0 inches. If you use a different rim width, the section width will change slightly—wider rims stretch the tire, making the section width larger, while narrower rims make it smaller. For practical purposes, you can measure the section width on your mounted tire with a tape measure, but be aware that the actual measured width may differ from the sidewall number by a few millimeters due to rim width, inflation pressure, and load.

How to Measure Tire Tread Width

Tread width is the distance between the outer edges of the tread pattern, often less than the section width. It is not typically listed on the tire sidewall; you must measure it directly. To measure tread width, place a tape measure across the tread face from one outer edge of the tread blocks to the other. Tread width varies by tire design; performance tires may have tread widths close to section width, while touring tires may be narrower. Knowing the tread width can help you understand the tire's footprint and potential traction characteristics.

How to Measure Tire Width and Height Overall

To physically measure the mounted tire's overall width and height (diameter):

1. Ensure the tire is inflated to the vehicle manufacturer's recommended pressure.
2. For overall width: Use a tape measure or caliper to measure across the tire at its widest point, excluding any sidewall protrusions like raised lettering or rim protectors.
3. For overall height (diameter) when mounted on the vehicle: Measure from the ground to the top of the tire (at the center of the tread) with the vehicle's weight on the tire. This is the loaded radius; the diameter is twice that.
4. For overall diameter when off the vehicle: You can measure the circumference with a flexible tape, then divide by π (3.1416) to get the diameter. Alternatively, measure the radius from the center of the hub to the outermost point of the tread and multiply by two.

Keep in mind that the actual measured diameter may differ from the calculated diameter due to tire deflection under load, manufacturing tolerances, and wear. The calculated diameter from sidewall markings is the theoretical diameter of an unloaded, properly inflated tire on its measuring rim.

Calculating Tire Dimensions from Sidewall Markings

You can calculate several dimensions using the sidewall markings:

• Sidewall height = (Section width in mm × Aspect ratio) / 100
• Overall diameter in inches = (Sidewall height × 2 / 25.4) + Rim diameter (in inches)
• Circumference = Overall diameter × π (3.1416)
• Revolutions per mile = 63,360 / Circumference (in inches)

Example: For P225/50R17:

• Sidewall height = 225 × 50 / 100 = 112.5 mm
• Overall diameter = (112.5 × 2 / 25.4) + 17 = (225 / 25.4) + 17 = 8.86 + 17 = 25.86 inches
• Circumference = 25.86 × 3.1416 ≈ 81.2 inches

These calculations are essential for comparing tire sizes when considering plus-sizing or ensuring speedometer accuracy.

Comparing Tire Sizes: Width, Aspect Ratio, and Rim Diameter

When changing tire sizes, it's important to understand how different dimensions affect the tire's overall size. Here's a table showing some common tire sizes and their calculated dimensions:

As you can see, changing the width, aspect ratio, or rim diameter can result in tires with similar overall diameters. For example, 225/50R17 and 225/45R18 have nearly the same overall diameter (25.9 vs 26.0 inches). This is because the larger rim is compensated by a shorter sidewall. When replacing tires, it's generally recommended to keep the overall diameter within about 3% of the original to avoid affecting speedometer and odometer readings.

Motorcycle Tire Measurement

Motorcycle tire sizes use a slightly different system. Many modern motorcycles use metric sizes similar to car tires, but some older bikes use inch-based sizes. For example, a common sport bike rear tire is 180/55ZR17, where 180 is the section width in mm, 55 is the aspect ratio, Z is the speed rating (over 149 mph), R is radial, and 17 is the rim diameter in inches. However, some cruisers use sizes like 130/90-16, where the tire is bias-ply (indicated by the dash instead of an R). To measure a mounted motorcycle tire's width and height, the same principles apply: read the sidewall for nominal section width and aspect ratio, then calculate or physically measure overall diameter.

Important Considerations for Measuring Mounted Tires

• Inflation pressure: Tire dimensions are specified at a certain inflation pressure. Underinflated tires will be wider at the bottom and have a smaller effective radius.
• Load: The weight of the vehicle compresses the tire, reducing its radius. The loaded radius is always less than the unloaded radius.
• Rim width: The tire's section width changes with rim width. A tire mounted on a wider rim will have a slightly larger section width.
• Wear: As a tire wears, its overall diameter decreases slightly due to tread wear.
• Manufacturing tolerances: Actual dimensions may vary from the nominal sidewall markings by a few percent.

Understanding these factors will help you get accurate measurements and make informed decisions about tire replacement and fitment.