Reading wheel and tyre sizes

• Module: Dimensions
• Level: Intermediate
• Reading time: 10 min

A wheel marked 8.5Jx19 H2 ET45 5x112 66.6 reads from left to right: 8.5 inches of width between the flanges, a flange with a J profile, a one-piece rim (the “x”), a diameter of 19 inches, two safety bumps (H2), an offset of 45 mm, 5 holes on a circle of 112 mm and a centre hole of 66.6 mm. A tyre marked 245/35 R19 93Y XL reads the same way: 245 mm wide, a sidewall as high as 35% of that width, a radial structure, a 19-inch rim, a load index of 93 (650 kg), a speed symbol Y (300 km/h) and a reinforced version.

These two lines are the starting point for knowing whether a wheel and a tyre go together, and whether they suit your car. Here is each marking, according to the UN regulations that set them, along with the reading mistakes found even on specialist websites.

How do you read the markings on a wheel?

The order varies from one manufacturer to another: “8.5Jx19” or “19x8.5J” refer to the same wheel. The sure way to tell: the number attached to the J is the width; the other one is the diameter.

An official example, decoded

The UN regulation on replacement wheels (No. 124), taken over into European law, itself gives an example of a marking: ABCDE 5½ J x 14 FH 36 01 99 ab123. It reads as follows: manufacturer ABCDE; 5.5-inch rim, J profile; one piece (x); diameter 14 inches; one flat hump (FH); offset of 36 mm; date of manufacture January 1999; part number ab123.

The same regulation sets what a replacement wheel must carry, legibly and visibly once the tyre is fitted: the manufacturer's name, the rim designation (profile and diameter), the offset, the date of manufacture (at least the month and the year) and the part number. The PCD and the centre bore are not part of this mandatory list.

Where do you find the size of your wheel?

The measurements are engraved or cast into the wheel, rarely on the front. Look:

• on the back of the spokes or between them, on the inner side;
• around the mounting area, on its outer or inner face;
• inside the barrel, the cylindrical part hidden by the tyre.

No need to take anything off to know the diameter: it is written on the sidewall of the tyre, just after the R. An “R19” tyre is fitted on a 19-inch wheel.

For your car's original sizes, look at the label on the driver's door pillar or on the fuel flap, the owner's manual, and above all the European conformity document (the CoC) supplied with the car: its item 35 gives the specified wheel and tyre combinations. Do not look for them on the registration document: the European list of its fields contains none for tyres, and box P.6, often quoted, is the administrative horsepower rating.

What do the J, the x and the H2 mean?

The J refers to the flange profile, its standardised shape. The ETRTO, the European body that standardises tyres and rims, defines basic B and J profiles for car rims; J is the one used in official examples as well as in manufacturers' data sheets (Audi writes “10.5Jx22”, Volkswagen “7.0 J x 17”). Contrary to what you sometimes read, the J refers neither to a stiffness nor to a hump.

The x means, under Regulation No. 124, that the rim in the technical sense, the hoop that carries the tyre, is in one piece; the ISO 3911 standard specifies that a one-piece rim has a well, the hollow used for fitting the tyre. A dash in place of the x means a rim made of several pieces.

The H2 indicates two humps, those small bumps next to the bead seats that hold the tyre if it loses pressure. Other codes refer to other shapes or layouts: FH, for example, is a flat hump on one side only. Several websites present H2 as a “stiffness” of the wheel: that is wrong.

Why does a 19-inch wheel measure more than 48 cm?

Because the diameter written on it is a nominal diameter, that of the bead seats, where the tyre rests. 19 inches make 19 × 25.4 = 482.6 mm. The flanges stand proud above these seats: measured edge to edge, the wheel is therefore larger. The same goes for the width, taken between the flanges: an 8.5-inch wheel (215.9 mm) measures more overall, by about 25 mm.

This is the most common cause of discrepancies when someone measures a wheel themselves with a tape measure.

How do you read a tyre? The example of 245/35 R19 93Y XL

The load index and the speed symbol are read together: 93Y. They must be at least equal to those the manufacturer lists for the car. The values come from tables set by the UN regulation on tyres (No. 30):

What other markings are found on a tyre?

• The date of manufacture: a four-digit code at the end of the number that follows “DOT”. The first two give the week, the last two the year: “1224” means the twelfth week of 2024.
• ZR in place of R (245/35 ZR19): permitted for W and Y tyres, mandatory above 300 km/h, the index then being written in brackets, “(95Y)” for example.
• M+S: a tyre whose tread pattern is designed for mud and snow, with no grip test required. The pictogram of a three-peaked mountain with a snowflake, on the other hand, signals a tyre whose grip on snow has been measured using a regulatory method.
• The E marking: an E in a circle followed by the number of the country that issued the tyre's approval (2 for France), then an approval number.
• TWI: the markers of the wear indicators, small blocks at the bottom of the grooves, at 1.6 mm. In France, this is the legal minimum depth of the main grooves of a car tyre.
• The direction of rotation (“Rotation” and an arrow) or the fitting side (“Outside”) on directional or asymmetric tyres, and a marking specific to run-flat tyres (ZP at Michelin, SSR at Continental, for example).

The European label, with its classes from A to E for fuel consumption and wet grip and its noise in decibels, is not marked on the tyre: it is a sales label.

How do you calculate the overall diameter of a wheel?

You add the rim diameter and twice the sidewall height, as Regulation No. 30 does:

overall diameter = rim diameter × 25.4 + 2 × width × aspect ratio ÷ 100

For a 245/35 R19: 482.6 + 2 × 85.75 = 654.1 mm. For a 225/40 R18: 457.2 + 2 × 90 = 637.2 mm. The overall diameter matters because the speedometer and the room under the wing depend on it; in Belgium, a tyre of a size other than the specified one must stay between −2% and +1.5% of the specified diameter. The tyre size calculator compares two sizes and gives the difference.

Do a wheel and a tyre go together?

Read the two lines side by side. With an 8.5Jx19 wheel and a 245/35 R19 tyre:

1. The diameter: 19 and R19 match. It is the only rule with no exception.
2. The width: each tyre size accepts a range of rim widths, published by tyre manufacturers and the ETRTO. A rim that is too narrow pinches the tyre; too wide, it stretches it.
3. The indices: load and speed at least equal to those the manufacturer lists for the car.

A real case, taken from Audi's genuine accessories catalogue: a complete winter wheel for the RS 6 Avant combines a 10.5Jx22 wheel, PCD 112/5, offset 19 mm, and a 285/30 R22 101W XL tyre. You can now read each of these figures.

The most frequent reading mistakes

• Reading “R” as “radius”. R means radial. The 19 that follows is a diameter.
• Confusing the width of the wheel with that of the tyre. The wheel is measured in inches (8.5), the tyre in millimetres (245).
• Taking “35” for a height in millimetres. It is a percentage of the width.
• Taking H2 for a stiffness. H2 means two humps.
• Confusing ET and PCD. ET45 is an offset; 5x112 is the PCD.
• Measuring the wheel at the edge of the flanges. The diameter is taken at the bead seats, the width between the flanges.

The two subtlest measurements each have their own lesson: the offset (ET), which positions the wheel in the wing, and the PCD and centre bore, which fix it to the hub.

The Onyx approach — In a made-to-measure project, these dimensions are not chosen at random: from the diameter and the wheel position you have chosen, the Onyx Forged design office calculates the offset and, where needed, the width for your car as you described it to us (version, brakes, ride height, bodywork), then submits to you the measurements to be machined, before anything is made. Your approval covers these measurements: other tyres or a different ride height can change the position obtained.

Frequently asked questions

What does 7J mean on a wheel?

A wheel 7 inches wide between the flanges (177.8 mm), with a J flange profile.

How can I find out the size of my wheels without taking them off?

Read the sidewall of the tyre: the number after the R is the diameter of the wheel. For the width and the offset, you need the owner's manual, the CoC, the manufacturer, or the markings on the back of the wheel.

What is the difference between a 91H and a 91V tyre?

The same load (615 kg per tyre), but not the same maximum speed: 210 km/h for H, 240 km/h for V.

Can a 245/35 R19 be fitted on a 9-inch wheel?

Only if 9 inches is within the range of rims the tyre manufacturer permits for that tyre size. Check it for the exact size, with the tyre manufacturer.

In the next lesson, we take a closer look at the subtlest measurement on the line: the offset (ET).

Lesson written by the Onyx Forged design office.

This lesson is part of the Onyx Academy, the free course on custom forged wheels.

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• Wheel offset calculator

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