JATMA Tire Standards: Reading Sidewall Numbers, Rim Dimensions and Load-Inflation Tables

JATMA tire standards are the technical specifications published by the Japan Automobile Tyre Manufacturers Association. They define what a tire size designation such as 205/55R16 91V means in Japan. For each designated size, the standards set design dimensions, approved rim dimensions, and the load a tire may carry at a given inflation pressure. The main reference document is the annual JATMA Year Book. The numbers on a tire sidewall are the key to that book: they tell you which tables to look up and what can be compared between two tires.

What are JATMA tire standards?

JATMA tire standards are Japan's industry reference for tire dimensions, rims, valves and load capacities. They cover the following:

• tire size designations
• design section width and overall diameter, with permitted tolerances
• measuring rims and approved rim widths
• rim contour dimensions
• load-inflation tables

The standards apply to several tire categories, including passenger car, light truck, truck and bus, and motorcycle tires.

JATMA plays a role in Japan similar to the role of ETRTO in Europe and the Tire and Rim Association (TRA) in the United States. All three work within the international ISO framework. As a result, most metric tire sizes have the same nominal meaning in all three systems, although some details differ.

How do you read the numbers on a tire sidewall?

A metric tire size marking combines three kinds of information:

• dimensions, in millimetres and inches
• a ratio, as a percentage
• performance ratings, as coded index numbers and letters

Take 205/55R16 91V as an example. Each part has a different unit and purpose.

Marking	Type	Meaning	Unit
205	Dimension	Nominal section width	Millimetres
55	Ratio	Aspect ratio: sidewall height as a share of section width	Percent
R	Construction	Radial construction	None (code)
16	Dimension	Nominal rim diameter	Inches
91	Performance rating	Load index: 91 = 615 kg maximum load per tire under specified conditions	Index number
V	Performance rating	Speed symbol: V = 240 km/h under specified conditions	Letter code

Dimensions and ratios: width, aspect ratio and rim diameter

Section width. The first number is the nominal section width in millimetres. It is measured across the tire's widest point, excluding raised lettering and protective ribs, when the tire is mounted on its measuring rim.

Aspect ratio. The aspect ratio is not a length. It is a percentage: 55 means the sidewall height is about 55% of the section width.

Rim diameter. The number after the construction letter is the rim diameter, given in inches even though the width is in millimetres. This mix of units is a normal part of metric tire size designations.

Construction code

The letter R means radial construction. Older or specialised tires may show a hyphen instead, as in 6.50-16, which indicates bias construction. Bias sizes such as this one express width in inches and often carry no aspect ratio.

Performance ratings: load index and speed symbol

The load index and speed symbol are codes, not measurements. Under JATMA tire standards and the matching international system:

• Load index 91 corresponds to a maximum load of 615 kg per tire.
• Speed symbol V corresponds to 240 km/h.

Both values apply only under the conditions defined in the standards, including correct inflation. They describe the tire's capability. They are not a recommendation to drive at that speed or load.

A higher load index means more capacity. Two tires of the same size can have different load indexes.

Light truck, extra load and reinforced markings

Light truck tires. These often carry two load indexes, as in 195/80R15 107/105L LT:

• 107 (975 kg) is the load per tire when the tire is fitted singly.
• 105 (925 kg) is the load per tire in a dual fitment.
• L is the speed symbol, corresponding to 120 km/h.

Extra load tires. Passenger tires marked XL, EXTRA LOAD or REINFORCED belong to a different load-inflation table than standard load tires. They reach their higher rated load at a higher reference pressure.

Japanese tire labeling grades are not size markings

JATMA also runs a voluntary tire labeling system for passenger car replacement tires in Japan. It grades two things:

• Rolling resistance: AAA, AA, A, B or C
• Wet grip: a, b, c or d

These grades are performance ratings, usually shown on a product label. They are not part of the dimensional size designation and say nothing about fitment.

How do width, aspect ratio and rim diameter determine tire geometry?

Nominal tire geometry can be calculated from the three dimensional parts of the size designation. The formulas are:

• sidewall height (mm) = section width (mm) × aspect ratio ÷ 100
• overall diameter (mm) = rim diameter (inches) × 25.4 + 2 × sidewall height
• circumference (mm) = π × overall diameter

Step-by-step calculation for 205/55R16

1. Find the sidewall height: 205 × 0.55 = 112.75 mm.
2. Convert the rim diameter: 16 × 25.4 = 406.4 mm.
3. Add both sidewalls to the rim diameter: 406.4 + 2 × 112.75 = 631.9 mm overall diameter.
4. Find the circumference: 3.1416 × 631.9 ≈ 1,985 mm.
5. Find the revolutions per kilometre, if needed: 1,000,000 ÷ 1,985 ≈ 504.

How each number changes the result

Width. Increasing the width while keeping the same aspect ratio also increases the sidewall height, because the ratio is a percentage of the width.

Aspect ratio. Lowering the aspect ratio shortens the sidewall.

Rim diameter. Increasing the rim diameter adds 25.4 mm to the overall diameter for every inch. That is why a larger rim usually requires a lower aspect ratio to keep the overall diameter similar.

Size	Sidewall height	Overall diameter	Circumference	Diameter vs 205/55R16
205/55R16	112.75 mm	631.9 mm	≈ 1,985 mm	Reference
225/45R17	101.25 mm	634.3 mm	≈ 1,993 mm	+2.4 mm (≈ +0.4%)
215/60R16	129.0 mm	664.4 mm	≈ 2,087 mm	+32.5 mm (≈ +5.1%)
195/65R15	126.75 mm	634.5 mm	≈ 1,993 mm	+2.6 mm (≈ +0.4%)

The table shows that 205/55R16, 225/45R17 and 195/65R15 have almost the same calculated overall diameter, even though their widths and rim sizes differ. In contrast, 215/60R16 is about 5% larger in diameter on the same 16-inch rim, because both the width and the aspect ratio increased.

A smaller example is a typical Japanese kei car size, 155/65R14:

• sidewall height: 155 × 0.65 = 100.75 mm
• overall diameter: 355.6 + 201.5 = 557.1 mm

What a diameter difference means for speed and distance readings

The percentage difference in circumference is the same as the percentage difference in diameter. If a speedometer was calibrated for 205/55R16 and the vehicle is fitted with 215/60R16, each wheel revolution covers about 5.1% more distance.

As a result, at an indicated 100 km/h the calculated true speed would be about 105 km/h. The odometer would under-record distance by a similar proportion. This is a geometric estimate. Actual speedometer behaviour also depends on how the vehicle was calibrated.

How does calculated geometry differ from actual tire dimensions?

Calculated geometry gives nominal values. Actual tire dimensions can differ for several reasons.

Design dimensions in the tables. JATMA tire standards list a design section width and design overall diameter for each size. These are measured on a specified measuring rim at a specified inflation pressure. The tables also set maximum and minimum allowable values, so tires of the same size from different manufacturers can measure slightly differently.

Rim width. Section width depends on the rim. Mounting a tire on a rim wider or narrower than the measuring rim changes the measured section width. A commonly used rule of thumb is a change of roughly 5 mm for each 0.5-inch change in rim width.

Inflation and load. Under load, the tire deflects. Its effective rolling circumference is therefore smaller than the circumference calculated from the free, unloaded diameter.

Tread wear. Wear reduces the overall diameter by about twice the tread depth lost. For example, losing 6 mm of tread depth reduces the diameter by about 12 mm.

Tread design. Winter tires, all-terrain tires and tires with protective rim ribs or tall tread blocks may measure differently from summer tires of the same designation.

To compare tires accurately, use the design dimensions in the current standard and the manufacturer's published specifications, not only the calculation from the sidewall numbers.

What do rim dimensions in JATMA tables tell you?

The rim dimension tables in JATMA tire standards define which rim widths and contours a tire size is designed to use. A rim marking such as 16x6.5J means:

• 16-inch nominal diameter
• 6.5-inch rim width, measured between the flanges
• J flange profile

For each tire size, the standards identify a measuring rim, on which the design dimensions are measured. They also list a range of approved rims.

Rim diameter must match exactly: a 16-inch tire fits only a 16-inch rim of the matching contour. Rim width, by contrast, can vary within the approved range. If a proposed wheel width is outside that range, the tire is not approved for that rim in the standard, even if it can physically be mounted.

Wheel offset, which Japanese wheel markings usually call inset, and the bolt pattern are vehicle and wheel fitment matters. The tire size designation does not describe them.

How do JATMA load-inflation tables work?

A load-inflation table shows how much load a tire of a given size and load category can carry at each inflation pressure. The pressure is given in kilopascals (kPa).

The load index on the sidewall shows the maximum load, which applies at the table's specified pressure for that tire type. At lower pressures, the permitted load is lower.

The tables matter most in three situations:

• Changing size. A replacement tire must carry the vehicle's axle loads at a pressure the vehicle can use. Check its load capacity at the intended pressure, not only its load index.
• Standard load vs extra load. Extra load tires follow separate tables. They do not carry more load than a standard load tire of the same size at the same low pressure. Their advantage appears at higher pressures.
• Light truck tires. Single and dual loads differ, and light truck tires typically run at higher pressures than passenger tires.

The vehicle's tire placard or owner's manual remains the reference for the operating pressure on that vehicle. The load-inflation tables explain the tire's capacity behind that pressure.

How do JATMA standards compare with ETRTO and TRA?

JATMA, ETRTO and TRA standards use closely aligned metric tire size designations. A passenger tire marked 205/55R16 91V has the same nominal dimensional meaning and the same load index and speed symbol values in all three systems.

The differences lie mainly in two areas:

• Reference pressures. The reference pressures in the load-inflation tables and some table values can differ slightly between organisations.
• Market-specific designations. Examples include P-metric and flotation sizes in North America, the C (commercial) suffix in Europe, and LT conventions used in Japan.

When precise load or pressure figures matter, use the table from the standard that applies to the tire and the vehicle, and do not mix values between organisations.

Where can you find JATMA tire standards and a JATMA tyre standards PDF?

The primary reference document for JATMA tire standards is the JATMA Year Book. It is published annually by the Japan Automobile Tyre Manufacturers Association and contains, by tire category:

• size designations
• dimension tables
• rim dimensions
• load-inflation tables
• related technical definitions

The Year Book is a copyrighted publication distributed by JATMA. People searching for a JATMA tyre standards PDF often find excerpts or older editions online. These may not match the current edition, because sizes, tolerances and tables are revised over time.

To use a JATMA reference document correctly, follow these steps:

1. Check the edition year of the Year Book or excerpt, and prefer the current edition from JATMA.
2. Identify the tire category from the sidewall: passenger car, light truck (LT), truck and bus, or another category.
3. Find the exact size designation in the dimension table for that category.
4. Read the design section width, design overall diameter and allowable tolerances.
5. Note the measuring rim and the approved rim widths.
6. Open the load-inflation table for the correct load category (standard load, extra load, or light truck single/dual), and read the load at the pressure you plan to use.
7. Compare the results with the vehicle's placard values and the tire manufacturer's specifications.

What a size comparison cannot tell you about vehicle fitment

Calculated geometry and JATMA table values show whether two tire sizes are dimensionally similar. They do not confirm that a tire fits a particular vehicle.

Fitment also depends on several factors that the size designation does not cover:

• clearance to suspension parts, brakes, fenders and body panels through full steering and suspension travel
• wheel width, offset and bolt pattern
• the vehicle's required load index and speed symbol
• electronic systems calibrated to rolling circumference
• local vehicle regulations

A small calculated diameter difference, such as the 0.4% between 205/55R16 and 225/45R17, does not guarantee clearance for a tire that is 20 mm wider. A matching load index does not guarantee adequate capacity at the pressure the vehicle actually uses.

Use the sidewall numbers and JATMA tire standards to understand and compare sizes. Then confirm fitment against the vehicle maker's approved sizes and specifications.