Euro-metric and P-metric tire markings: format and replacement questions

The short answer: what a Euro-metric size means in inches

A Euro-metric tire size such as 225/45R17 mixes two unit systems. The first number is the tire's section width in millimeters. The second is the aspect ratio, which is the sidewall height expressed as a percentage of that width. The letter R indicates radial construction. The last number is the wheel (rim) diameter, and that one is already in inches.

To turn the metric parts into inches, divide millimeters by 25.4. For 225/45R17, the width is 225 mm, or about 8.86 inches. The sidewall height is 45 percent of 225 mm, or about 101 mm, which is close to 4 inches. The approximate overall diameter is the 17-inch rim plus two sidewalls, or about 25 inches.

A Euro-metric size has no letter in front of the width. A P-metric size uses the same layout but starts with a P, as in P225/45R17. The dimensions are read the same way. However, the two types are not automatically interchangeable, because they can be rated for different loads at the same inflation pressure. A size code tells you dimensions. It does not tell you whether a tire fits a particular vehicle.

How to read each part of the marking

Using 225/45R17 91V as an example:

• 225 is the section width in millimeters, measured across the widest point of the tire on a specified measuring rim.
• 45 is the aspect ratio. Sidewall height equals 45 percent of the section width.
• R means radial construction.
• 17 is the rim diameter in inches. The tire must be mounted on a wheel of this diameter.
• 91 is the load index, a code that corresponds to a maximum load per tire in a standardized table. It is not a weight in pounds or kilograms.
• V is the speed rating, a letter code for the maximum sustained speed the tire is rated for under specified conditions.

Tire makers generally list sizes in this order: width, then sidewall height as a percentage, then the construction letter, then rim diameter. Some Euro-metric tires also carry a marking such as XL or Extra Load. This indicates a reinforced version that is rated for a higher load than the standard-load version of the same size.

Converting the size to inches

The conversion is simple arithmetic. Because the sidewall height depends on both the width and the aspect ratio, a lower aspect ratio does not always mean a shorter sidewall when you compare tires of different widths.

Width (in)          = width_mm / 25.4
Sidewall height (in) = (width_mm x aspect_ratio / 100) / 25.4
Overall diameter (in) = rim_in + 2 x sidewall_height_in

Example: 225/45R17
Width          = 225 / 25.4              = 8.86 in
Sidewall       = (225 x 0.45) / 25.4     = 3.99 in
Overall diameter = 17 + (2 x 3.99)       = 24.97 in

These results are nominal figures calculated from the size code. Actual measured width and diameter vary with the tire model, the tread design and the width of the wheel the tire is mounted on. If clearance matters, use the published dimensions for the specific tire. Do not rely on the calculated value alone.

Why tire sizes mix millimeters and inches

Earlier tire sizing was expressed entirely in inches. A Ford Model A, for example, might use a 4.75-19 tire, which fits a 19-inch wheel. When metric sizing was adopted for width and sidewall proportion, wheel diameters remained in inches. Today's codes combine metric width, a percentage-based sidewall and an inch-based rim size. Some light-truck and off-road tires still use all-inch formats that state overall diameter and width directly, so not every size on the market follows the metric layout.

P-metric versus Euro-metric: what differs

Feature | P-metric | Euro-metric

Prefix | Starts with P (for example P225/45R17) | No letter before the width (for example 225/45R17)

How dimensions are read | Width in mm / aspect ratio / construction / rim diameter in inches | Same layout and same reading

Load rating | Defined by its own load and inflation standard | Defined by a separate standard; available in standard-load and reinforced or extra-load versions

Same numbers, same load capacity? | Not guaranteed | Not guaranteed; check the load index and the inflation pressure required to carry it

The main practical point is that matching numbers do not guarantee matching load capacity at a given tire pressure. The industry publishes separate load and inflation tables for P-metric and Euro-metric tires. When you switch from one type to the other, the inflation pressure needed to carry the vehicle's load may change. The exact pressure depends on the specific size and load index. This guide does not supply those tables. Use the tire manufacturer's published data or ask a tire professional.

Steps to check a replacement between P-metric and Euro-metric

Before you start, gather the vehicle's tire information placard (commonly on the driver's door jamb), the owner's manual, the sidewall markings on the current tires, and the specification sheet for any tire you are considering. Treat the vehicle manufacturer's specification as the baseline. The steps below explain how to compare tires. They do not replace instructions for a specific vehicle.

• 1. Record the original equipment size, load index, speed rating and recommended cold inflation pressures from the placard. Note whether the original size is P-metric or Euro-metric.
• 2. Read the full marking on the candidate tire, including any P prefix, the load index, the speed rating and any XL or Extra Load designation.
• 3. Confirm the dimensions match the original size or an alternative the vehicle manufacturer approves. Do not infer fitment from the code alone. Wheel width, offset, suspension and body clearance, and powertrain requirements can rule out a size that looks close on paper.
• 4. Compare load-carrying capacity at the pressure you will actually run, not only the load index number. If you are moving between P-metric and Euro-metric, look up both tires in the applicable load and inflation tables or ask the tire maker. Stop if the replacement cannot carry at least the load the original tire was specified to carry.
• 5. Compare speed ratings. Many manufacturers advise against fitting a lower speed rating than the original. Check the vehicle manufacturer's guidance for your model.
• 6. Determine the inflation pressure the replacement needs. If it differs from the placard value, get the correct figure from the tire manufacturer or a qualified tire dealer. Do not guess.
• 7. Have the tires mounted and balanced by a qualified tire service. Mounting, bead seating, balancing, tire pressure monitoring system (TPMS) service and wheel torque all require proper equipment and the vehicle's specifications.
• 8. Completion check: after installation, verify the cold pressures against the figure you confirmed, check that the TPMS does not show a warning, and recheck wheel fastener torque if the installer recommends it.

Do not mix tire types or sizes on the same axle unless the vehicle manufacturer permits it. This matters especially on all-wheel-drive and four-wheel-drive vehicles, which can be sensitive to differences in rolling diameter. The manufacturer's guidance for your model year, market and drivetrain overrides any general rule.

Limits of this guide

This guide explains how to read the size format and convert it to inches. It does not give load or inflation values for any specific size, does not confirm compatibility with any vehicle, and does not cover regional legal requirements for tire fitment. For those questions, consult the vehicle manufacturer's documentation, the tire maker's published specifications or a qualified tire professional.