Michelin TRX Tire Sizes: Metric Markings, Specifications and Inch Conversions

Michelin TRX tire sizes are fully metric. A marking such as 190/65 R 390 describes four things. The tire has a 190 mm section width and a 65% aspect ratio. It uses radial construction. It fits a rim with a nominal diameter of 390 mm, which is about 15.35 inches. A 390 mm TRX rim is neither a 15-inch nor a 16-inch rim. Because of this, TRX tires mount only on matching TRX metric wheels, and inch-sized tires cannot be fitted to TRX wheels.

The sections below explain how to read the numbers on a TRX sidewall and how to convert them to inches. They also show how to calculate sidewall height, overall diameter and circumference, and why those calculated figures are not the same as measured size or proof of vehicle fitment.

How do you read a Michelin TRX size on the sidewall?

A Michelin TRX size marking follows the order section width / aspect ratio, construction, rim diameter. Every dimension is given in millimeters or as a percentage. Nothing is given in inches. Period tires often place a speed-category letter just before the R, as in 190/65 HR 390 or 240/45 VR 415.

Step-by-step: decoding a TRX size

1. Read the first number (section width). In 190/65 R 390, the 190 means the tire's nominal section width is 190 millimeters. That is the width from sidewall to sidewall at the widest point, measured on a specified rim, not the width of the tread.
2. Read the number after the slash (aspect ratio). The 65 means the sidewall height is 65% of the section width. It is a ratio, not a measurement in millimeters.
3. Identify the letters in the middle. R means radial construction. A letter directly before the R, such as H or V, is an older-style speed category built into the size.
4. Read the last number (metric rim diameter). The 390 is the nominal rim diameter in millimeters. On a conventional tire this number would be 15 or 16, in inches. On a TRX tire it is a three-digit millimeter value.
5. Look for the TRX name. The TRX designation on the sidewall shows the tire is built for the TRX metric rim system and is not a standard inch-rim tire.

Example breakdown: 190/65 HR 390

What is a TRX tire size in inches?

To convert a TRX tire size to inches, divide each millimeter value by 25.4. For example, a 390 mm TRX rim diameter equals 390 ÷ 25.4 ≈ 15.35 inches. A 190 mm section width equals about 7.48 inches. The aspect ratio is a percentage, so it needs no conversion.

TRX rim diameters converted from millimeters to inches

The metric rim diameters below are the ones most often seen on TRX applications. Each one falls between two standard inch sizes. That is why a TRX tire cannot be swapped onto an inch wheel of the "nearest" size.

A 390 mm TRX rim is about 9 mm larger than a 15-inch rim and about 16 mm smaller than a 16-inch rim. A tire bead must seat on a rim of exactly the diameter it was designed for. Gaps of this size therefore make the two systems incompatible, not just approximately different.

TRX tire specs: calculated dimensions of common sizes

You can calculate the basic geometry of any TRX tire from its size marking with three formulas. All values are in millimeters until the final conversion.

• Sidewall height = section width × (aspect ratio ÷ 100)
• Overall diameter = rim diameter + (2 × sidewall height)
• Circumference = overall diameter × π (about 3.1416)

Worked example for 190/65 R 390:

1. Sidewall height: 190 × 0.65 = 123.5 mm (about 4.86 in).
2. Overall diameter: 390 + (2 × 123.5) = 637 mm (about 25.08 in).
3. Circumference: 637 × 3.1416 ≈ 2,001 mm (about 78.79 in).
4. Revolutions per mile: 1,609,344 mm ÷ 2,001 mm ≈ 804.

Calculated geometry for TRX-format sizes

The table applies the same formulas to several TRX designations. The figures are nominal calculations from the size marking only. They are not measured dimensions of any particular tire.

How do width, aspect ratio and rim diameter affect TRX tire dimensions?

In any TRX size, sidewall height depends on both section width and aspect ratio. Overall diameter then depends on sidewall height plus rim diameter. Changing any one of the three numbers changes the calculated overall diameter and circumference.

Section width

When the aspect ratio stays the same, a wider tire also has a taller sidewall. This is because the ratio is a percentage of the width. Compare 240/45 R 415 and 280/45 R 415. Both use a 45% ratio on a 415 mm rim. The extra 40 mm of width adds 18 mm of sidewall height (108 mm vs. 126 mm), which adds 36 mm to the overall diameter (631 mm vs. 667 mm).

Aspect ratio

A lower aspect ratio gives a shorter sidewall relative to width. 190/65 R 390 and 220/55 R 390 share a 390 mm rim. The 220 mm tire is wider, but its lower 55% ratio gives a sidewall of 121 mm. That is slightly shorter than the 123.5 mm sidewall of the 190/65. As a result, the wider tire has a slightly smaller calculated overall diameter: 632 mm vs. 637 mm.

Rim diameter

A larger metric rim diameter can be balanced by a shorter sidewall. 220/55 R 390 and 240/45 R 415 differ by 25 mm in rim diameter, yet their calculated overall diameters are almost identical at 632 mm and 631 mm. The 415 mm size reaches that diameter with a much shorter sidewall: 108 mm vs. 121 mm.

Why circumference matters

Circumference is the distance a tire travels in one revolution. In percentage terms, a difference in overall diameter is the same as the difference in circumference. A tire with a 1% larger diameter covers about 1% more distance per revolution. This affects speedometer and odometer readings and effective gearing.

How does a TRX size compare with an inch-sized tire?

A TRX size can be compared with an inch size using the same formulas, provided the inch rim diameter is converted to millimeters first (rim inches × 25.4). The comparison shows only how the calculated geometry differs. It does not mean the tires are interchangeable, because the two need different wheels.

Example: 190/65 R 390 compared with two conventional 15-inch sizes (15 in = 381 mm).

To estimate the effect on a speedometer calibrated for the TRX size, multiply the indicated speed by the new overall diameter divided by the original. With the 205/60 R15 example, an indicated 60 mph would correspond to about 60 × (627 ÷ 637) ≈ 59 mph actual. A smaller tire makes the speedometer read higher than true speed, and a larger tire makes it read lower.

What do the performance markings on a TRX tire mean?

Performance markings on a TRX tire describe rated capability, not physical size. They should be read separately from the dimensional numbers.

• Speed category inside the size. Older TRX markings place the speed letter before the R, as in HR or VR. H indicates a maximum of 210 km/h (about 130 mph). In the older format, VR indicated suitability above 210 km/h, without a fixed upper figure in the marking itself.
• Service description. Some tires carry a load index number followed by a speed symbol after the size. The load index is a code for the maximum load per tire at a specified inflation. It is not a weight in kilograms or pounds. Look up its meaning in a load index table.
• Construction and other text. R confirms radial construction. Markings such as TL (tubeless) describe how the tire is built and used, not its dimensions.

When comparing TRX tires, compare the speed and load ratings against the vehicle's original specification. A tire with the same dimensions but a lower rating is not an equivalent replacement.

Calculated geometry vs. actual dimensions and TRX wheel compatibility

Calculated TRX tire specs are nominal values derived from the size marking. Actual dimensions and vehicle fitment depend on factors the size numbers cannot capture.

Why measured size differs from calculated size

• Rim width. Nominal section width assumes a specified measuring rim. A narrower or wider rim changes the width the tire actually has when mounted.
• Inflation and load. A mounted, inflated and loaded tire bulges and deflects. Its loaded radius is smaller than half the calculated overall diameter.
• Design and tolerance. Tread depth, tread pattern, casing design and manufacturing tolerance all mean two tires with the same marking can measure slightly differently. A worn tire is also smaller in diameter than a new one.

TRX wheel compatibility

A Michelin TRX tire requires a TRX wheel designed for the same metric rim diameter. The TRX system uses its own millimeter-based rim diameters and a rim and bead profile that differ from conventional inch rims.

• A TRX tire cannot be mounted safely on an inch rim, and an inch tire cannot be mounted safely on a TRX rim. The bead seat diameters do not match.
• A 390 mm TRX tire does not fit a 15-inch or 16-inch wheel. A 365 mm TRX tire does not fit a 14-inch or 15-inch wheel. A 415 mm TRX tire does not fit a 16-inch or 17-inch wheel.
• Switching a TRX-equipped vehicle to conventional tires means replacing the wheels, not just the tires.

What the numbers cannot tell you about fitment

Similar overall diameter does not prove that a tire and wheel combination will fit a vehicle. Fitment also depends on:

• wheel offset;
• wheel width;
• clearance to suspension, bodywork and brake components;
• steering lock;
• the load and speed ratings the vehicle requires.

Use the calculations to understand and compare TRX tire sizes. Confirm fitment against the vehicle's original specification for its TRX wheels.