Interpret passenger-car rim diameter, width and profile markings

A full passenger-car rim designation such as 7,5Jx17H2 packs four facts into one string. 7,5 is the nominal rim width (7.5 inches, with a comma as the decimal separator). J is the flange contour. 17 is the nominal diameter code. H2 means the bead seats carry double round humps. The x marks a one-piece well-base rim. Both numbers are nominal codes tied to reference points on a standardized rim drawing. You cannot get either one by measuring across the outside of the wheel. The order of the numbers also varies. Some wheel makers write diameter first, as in 18×8.5, so work out what each number stands for before you read its value.

Terms you need to read a rim marking

Nominal rim width: the inside distance between the two rim flanges, the raised lips that hold the tire beads at each edge.
Nominal diameter code: a number such as 16 or 17 that corresponds to a reference diameter defined at the bead-seat and flange datum on the rim drawing, not at the outer flange tips.
Flange contour code: a letter such as J or B that identifies the shape of the flange profile.
Hump code: a letter code such as H or H2 that identifies round humps formed on the bead seats.
Well-base rim: a one-piece rim with a recessed well in the middle and 5° tapered bead seats. In Continental's passenger-car rim data, this family is written with an x in the designation.

Decoding a full designation step by step

ALCAR uses 7,5Jx17H2 as its worked example. Read it from left to right. The number before the flange letter is the nominal width. The letter right after it is the flange contour. The x separates the width-and-flange part from the diameter. The number after the x is the diameter code, and anything after that describes the hump configuration.

Part of marking	Meaning	Value in 7,5Jx17H2
7,5	Nominal rim width between the inside flange faces	7.5 in
J	Flange contour designation	J contour
x	One-piece well-base rim	Well-base
17	Nominal rim diameter code	17
H2	Hump configuration	Double round humps

Diameter-first and width-first notation

Not every marking or listing uses the same order. Curva Concepts explains 18×8.5 as an 18-inch diameter followed by an 8.5-inch width, so diameter comes first. ALCAR's 7,5Jx17H2 puts width first. If you compare the two by position alone, you would read them backwards. In a full designation, the flange letter sits next to the width, which helps you identify that number. A plain two-number size has no letter to guide you, so check the wheel maker's own explanation of its notation before assigning width and diameter.

Nominal codes versus physical measurements

Continental's technical data book defines rim width, labeled A, between the inside reference faces of the flanges. For a 7½ J rim, A is 190.5 ±1.5 mm, which equals 7.5 × 25.4 mm. Measuring across the outer faces of the flanges gives a larger number, because the flange thickness is added on both sides.

Diameter works differently. In the same drawing, diameter code 17 corresponds to a reference diameter D of 436.6 mm, taken at the bead-seat and flange datum. Multiplying the code gives 17 × 25.4 = 431.8 mm, but that figure is only arithmetic on the code. It does not replace D, and it is not the outside diameter across the flange tips. A tape measure across the wheel's outer edge will not reproduce either number.

Wheel diagrams can also mislead. On ALCAR's wheel-profile diagram, the vertical diameter arrow shows the pitch circle diameter, which is the bolt circle through the wheel's mounting holes, not the nominal rim diameter. The center bore, offset, hump and flange are labeled separately. Keep each of those specifications in its own column, apart from the rim size.

Quantity	Where it is defined	Example value
Nominal width (A)	Between inside flange reference faces	7½ J: 190.5 ±1.5 mm
Diameter code	Rim designation	17
Reference diameter (D)	Bead-seat/flange datum on the rim drawing	436.6 mm for code 17
Code × 25.4	Arithmetic only, not a defined dimension	431.8 mm for code 17
Pitch circle diameter	Bolt-hole circle, separate from rim size	Listed separately by the wheel maker

J and B flange contours

The flange letter is part of the size, not a decorative suffix. Continental's passenger-car well-base data covers both J and B contours, and the drawing separates them by the flange-profile dimension labeled G. G is 17.3 ±0.6 mm for J and 14.1 ±0.6 mm for B. A 7½J rim and a 7½B rim share the same nominal width but have different flange profiles. Record the letter with every size you compare.

Hump codes

Hump codes describe round ridges formed on the bead seats. In Continental's data, H means a round hump on the outer shoulder only, and H2 means double round humps, one on each shoulder. In 7,5Jx17H2, the H2 therefore tells you both bead seats are humped.

Rim markings are not tire markings

Tire sizes use a different system, even though they share the diameter code. In Continental's example 205/55 R16, 205 is the tire's nominal section width in millimeters, 55 is the aspect ratio (sidewall height as 55% of section width), R means radial construction, and 16 is the rim diameter code the tire is built for. The 205 mm section width is a tire dimension, not the rim width. For comparison, 205 mm works out to about 8.07 inches, while a 7.5-inch rim has a nominal width of 190.5 mm. Only the diameter code is meant to match between tire and rim.

A rim size chart you can build yourself

A simple worksheet keeps each part of a marking separate and records where you found its definition. Use these columns:

Complete marking as printed
Manufacturer, document and edition used to decode it
Notation order (diameter-first or width-first)
Nominal width
Diameter code
Flange code
Construction symbol (such as x for well-base)
Hump code
Reference drawing for endpoint definitions
Outside dimensions, measured or listed separately
Bolt-circle (pitch circle) diameter, kept separate

Illustrative example: the two documented markings entered as separate rows. Values are filled in only where the source explains them.

Marking	Notation order	Nominal width	Diameter code	Flange	Construction	Hump
18×8.5	Diameter first (per Curva Concepts)	8.5 in	18	Not stated	Not stated	Not stated
7,5Jx17H2	Width first (per ALCAR)	7.5 in	17	J	x (well-base)	H2 (double round humps)

What a decoded marking does not settle

Decoding gives you the rim's nominal size, flange contour and hump type. It does not tell you whether a wheel fits a particular car. Offset, bolt circle, center bore and the approved tire and wheel combinations for your vehicle are separate questions, and the vehicle and wheel manufacturers' documentation answers them. The tolerances quoted here come from Continental's 2015–2016 technical data book. If you need exact dimensions for engineering or purchasing, confirm them against the current edition of the applicable rim standard.