Rim size explained
A wheel marking such as 7.5Jx17 ET45 5x112 looks cryptic but is just five measurements in a row.
Reading the marking
| Part | Means | In this example |
|---|---|---|
| 7.5 | Rim width in inches, bead seat to bead seat | 7.5 inches wide |
| J | Flange profile — the shape of the rim edge | J profile, the common passenger-car type |
| 17 | Rim diameter in inches | 17 inches |
| ET45 | Offset in millimetres from the centreline | 45 mm positive |
| 5x112 | Bolt pattern: stud count and circle diameter in mm | 5 studs on a 112 mm circle |
Where to find it
Usually cast into the back of a spoke or the inner barrel. On many wheels the offset is a separate stamp, so look for a standalone ET number if the main marking seems incomplete. Centre bore is often not marked at all and has to be measured.
J is not decoration
The letter after the width is the flange profile — the cross-sectional shape of the rim edge that the tyre bead seats against. Almost every modern passenger car wheel is J. You will also see JJ on some 4x4 and light commercial wheels, and B, P or D on older or specialist fitments. A tyre designed for a J flange should be fitted to a J or JJ rim; the profile affects how the bead is retained under cornering load.
You may also see an H or FH marking, meaning humped or flat-humped. Those humps sit just inboard of the bead seat and stop a tubeless tyre unseating if it loses pressure. Any wheel intended for tubeless use will have them.
Two markings people miss
Centre bore is usually not stamped anywhere and has to be measured with calipers across the hole in the middle. If it is larger than your hub, the wheel needs spigot rings so the hub carries the load rather than the bolts.
Load rating and construction date appear on the wheel too, often as a small stamp on the inner barrel. On a used wheel the date matters: an alloy that has been kerbed and straightened is not worth the saving.
Which must match
Bolt pattern and centre bore are vehicle-specific and non-negotiable. Diameter must match the tyre. Width has a usable range per tyre — see the tyre width chart. Offset is adjustable within the clearance you have, which is what the fitment calculator is for.
Related
How are these fitment figures calculated?
Fitment North calculates declared wheel and tyre geometry from the dimensions shown on the page, using the same tested method as the wheel-offset and tyre-size calculator. Tyre diameter uses rim diameter plus two calculated sidewalls; offset comparisons split width changes across inner and outer edges before applying the offset difference. Results are measurement aids, not vehicle approval. Confirm ratings, clearances and manufacturer guidance before fitting parts.