In short. The tyre is the rubber part: the tread that touches the road, the sidewalls and the beads that hold it on. The wheel is the metal part between the tyre and the axle. The rim is one part of the wheel: the outer ring the tyre is mounted on and supported by. The rest of the wheel, the centre with its spokes or disc, joins the rim to the car's hub. So every rim is part of a wheel, but a wheel is more than a rim. That is how the UN rule for replacement wheels and the ISO vocabulary define them, and the US rules for tyres say the same about the rim: the support on which the tyre's beads are seated. In everyday English, especially in the US, "rims" is often used for the whole wheel, usually an aftermarket one. That is fine in a shop, but when you read a size, it pays to know which part each number belongs to: in 225/40 R18 on an 8J x 18 wheel, the "18" is one and the same measurement, taken where the tyre's bead sits on the rim.
What the official definitions say
In our reading of the pages that came up first for this question on 15 October 2026, none quotes a definition, and they contradict each other: one counts the tyre as part of the wheel, another says the rim is only the outer lip. The texts that wheel and tyre makers actually work to are consistent with each other:
| Part | UN Regulation No. 124 and ISO 3911 (wheels) | UN Regulation No. 30 and US FMVSS 139 (tyres) |
|---|---|---|
| Wheel | "a rotating load-carrying member between the tyre and the axle", usually made of two major parts, the rim and the wheel disc | Not defined: the tyre texts only need the rim |
| Rim | "that part of the wheel on which the tyre is mounted and supported" | The support for a tyre (with or without a tube) "on which the tyre beads are seated" |
| Wheel disc (centre) | "that part of the wheel which is the supporting member between the axle and the rim" | Not defined |
| Tyre | Not defined: the wheel texts stop at the rim | Defined part by part: tread, sidewall, bead, carcass (below) |
Sources: UN Regulation No. 124, § 2.1, 2.1.3 and 2.1.4; ISO 3911:2021, 3.1, 3.2.1 and 3.2.2; UN Regulation No. 30, § 2.19; 49 CFR 571.139, S3.
Two consequences. The tyre is not part of the wheel: the wheel ends where the tyre's bead begins. And the "disc" of the standards is not the brake disc: it is the centre of the wheel, whether it is a solid disc, as on a steel wheel, or a set of spokes, as on most alloy wheels.
Why "rims" so often means wheels
UN Regulation No. 124 adds one sentence that explains the confusion: the rim and the wheel disc "may be integral, permanently attached, or detachable" (§ 2.1).
- Integral: a cast or forged alloy wheel formed as a single piece. The rim and the centre are the same block of metal, so there is no separate part anyone could hand you as "the rim". The word slid to the whole object.
- Permanently attached: a pressed steel wheel, whose rim and disc are two parts joined for good.
- Detachable: a multi-piece wheel, where the rim (often called the barrel) and the centre are separate parts held together by bolts. Here "rim" still means the rim, and a two-piece or three-piece wheel can have its rim replaced on its own. An American maker of forged wheels, for instance, describes its two-piece wheels as a one-piece forged barrel mated to a forged wheel centre (a wheel maker's product page).
The wheel's marking says something about the rim, not about the whole wheel. The rule asks for an "x" in the rim designation for a one-piece rim and a "-" for a multi-piece rim (§ 5.1.2.1): 8J x 18 has a one-piece rim, 8J-18 does not. In our reading, a two-piece wheel built on a one-piece barrel therefore still has a one-piece rim; the "-" belongs to a rim that is itself made of several parts, such as a barrel assembled from two halves. Our guide to the parts of a wheel takes the three constructions apart, piece by piece.
The tyre, part by part
UN Regulation No. 30 and the US standard FMVSS 139 define the parts of a tyre in almost the same words:
| Part of the tyre | Definition | What it does for you |
|---|---|---|
| Tread | The part that comes into contact with the ground (the road, in the US text) | Grip; it is what wears |
| Sidewall | The part between the tread and the bead | Carries the size and other markings; flexes with every turn |
| Bead | The part shaped to fit the rim and hold the tyre on it; the US text adds that it is made of steel wires wrapped or reinforced by ply cords | The joint between tyre and wheel |
| Carcass | The structure other than the tread and sidewall rubber which, when inflated, bears the load | Carries the car, with the air inside |
Sources: UN Regulation No. 30, § 2.4, 2.7, 2.8 and 2.9; 49 CFR 571.139, S3.
Where the tyre meets the rim
The meeting point has a name on each side. On the tyre it is the bead; on the rim it is the bead seat, which ISO 3911 defines as the part of the rim that gives the tyre radial support. Beside it, the flange is the part of the rim that gives the tyre lateral support (ISO 3911, 3.7.1 and 3.7.2). Air pressure pushes the bead outwards against the flange and down onto the seat, which is what makes a tubeless tyre airtight.
That joint is also where many slow leaks start. General Motors describes a tyre on an aluminium wheel that slowly goes flat because corrosion has formed at the bead seat, out of sight until the tyre is taken off (General Motors bulletin 08-03-10-006I). Our slow puncture guide covers the repair.
How a tyre size and a wheel size share one number
Here is a common pairing, read side by side. It illustrates the markings; it is not a size for your car.
| Marking | Belongs to | What each part means |
|---|---|---|
| 225/40 R18 | Tyre | 225 mm nominal section width; 40, the height of the sidewall as a percentage of that width; R, radial structure; 18, the nominal rim diameter in inches |
| 8J x 18 | Wheel (its rim) | 8, the rim width in inches, measured between the flanges; J, the shape of the flange; x, one-piece rim; 18, the rim diameter in inches |
The two 18s are the same measurement. UN Regulation No. 30 defines the tyre's nominal rim diameter as "the diameter of the rim on which a tyre is designed to be mounted" (§ 2.18), and on the wheel that diameter is taken at the bead seats, not at the outer lip. ISO 3911 marks the specified rim diameter on the bead seat in its rim drawing (Figure 9), and a German wheel maker puts it plainly: the rim diameter is the distance between the bead seats (a wheel maker's technical page). The outer edge of an 18-inch wheel is therefore a little larger than 18 inches, by the height of its flanges.
The width does not have to match one figure. A tyre has a range of permitted rim widths, set for each size from the ETRTO's values, not a single one (Continental, Technical Databook), and the US rule for passenger cars requires the rim to be one that the tyre maker lists as suitable for the tyre (49 CFR 571.110, S4.4.1). Our guide to reading a tyre size gives those ranges for common sizes.
How much of the height is wheel, and how much is tyre
Because the rim diameter is in inches and the tyre's section in millimetres, the overall diameter is easy to work out: the rim diameter plus twice the sidewall height, where the sidewall height is the width multiplied by the aspect ratio (UN Regulation No. 30, § 2.14 and 2.15; an inch is exactly 25.4 mm, NIST). Four examples, worked out by our design office from the nominal figures; a real tyre differs slightly from them:
| Tyre size (example) | Rim diameter | Sidewall height | Overall diameter | Share that is wheel |
|---|---|---|---|---|
| 225/45 R17 | 431.8 mm | 101.3 mm | 634.3 mm | 68 % |
| 225/40 R18 | 457.2 mm | 90.0 mm | 637.2 mm | 72 % |
| 235/35 R19 | 482.6 mm | 82.3 mm | 647.1 mm | 75 % |
| 245/30 R20 | 508.0 mm | 73.5 mm | 655.0 mm | 78 % |
The overall diameter changes by about 3 per cent from the first line to the last, while the wheel's share grows by ten points: that is how a bigger wheel usually goes on a car, with a lower tyre around it. Which sizes your car accepts is in its own documents: see what size wheels fit my car, and our tyre size calculator to compare two sizes.
The hub is not part of the wheel
The hub belongs to the car. It carries the wheel bearing, and the wheel bolts to it through its attachment face. The wheel has a centre bore, a hole in the middle that fits over a raised ring on the hub, which UN Regulation No. 124 calls the spigot (it speaks of the "spigot mounting diameter", § 2.4.4). The bolt holes around it sit on the pitch circle diameter, the PCD. A hubcap, wheel trim or centre cap is a cover, not part of the structure. How the bore and the PCD decide whether a wheel can be bolted on at all is in our guide to PCD, bolt pattern and centre bore.
Which part is at fault?
When something is wrong, the word matters, because the part you replace is not the same:
| What you see or feel | Part concerned | What the texts say |
|---|---|---|
| Tread worn down to the bars in the grooves | Tyre | Those bars are tread-wear indicators (UN Regulation No. 30, § 2.27). In Great Britain the tread must be at least 1.6 mm deep across the central three-quarters of its width, all the way round (regulation 27) |
| A cut or a bulge in the sidewall | Tyre | At the MOT, a cut deep enough to reach the ply or cords is a major defect; a lump, bulge or tear caused by a separation or partial failure of the structure is a dangerous one (5.2.3, MOT manual) |
| A tyre that sits unevenly on the rim | The joint: bead and rim | A tyre incorrectly seated on the wheel rim is a major defect (5.2.3) |
| Scuffs on the outer lip after touching a kerb | Rim (the flange) | Usually cosmetic: see alloy wheel repair |
| A slow leak with nothing in the tread | The joint: bead and bead seat, or the valve | GM's bulletin above: corrosion at the seat, found with the tyre off |
| A vibration at speed | Often the balance of the whole assembly | See wheel balancing |
| A bend in the rim after a pothole | Wheel | The MOT treats a badly distorted wheel as a major defect (5.2.2) |
| A crack anywhere on the wheel | Wheel | Any fracture or welding defect on a wheel is a dangerous defect at the MOT (5.2.2) |
| Wear at the centre bore | Wheel and hub | The MOT lists wear between wheel and hub at the spigot (5.2.2) |
Common mistakes
- Asking for "new rims" when you mean new tyres, or the reverse. A tyre fitter and a wheel maker are not the same trade.
- Reading the tyre's 18 as the wheel's outer diameter. It is the diameter at the bead seat; the lip is larger.
- Assuming a tyre fits only one width of rim. It fits a range, given in the tyre maker's data.
- Calling the hub part of the wheel. It stays on the car; only the wheel and tyre come off together.
- Reading the "x" in the marking as the sign of a one-piece wheel. The "x" or "-" describes the rim only: in our reading, a two-piece wheel built on a one-piece barrel carries an "x" too.
How it works at Onyx Forged
We make wheels, not tyres. You tell us the car and how you want the wheels to look; the Onyx Forged design office compares the rim width, diameter and offset you are considering with the dimensions in your car maker's documents, and goes through them with you before anything is made. Onyx Forged does not recommend a size or an offset: the car maker's documents remain the reference. Tyres are chosen and fitted by your tyre fitter, from the car maker's and the tyre maker's data.
Frequently asked questions
Is a rim the same as a wheel?
Not strictly. The rim is the outer part of the wheel, where the tyre is mounted; the wheel is the rim plus the centre that joins it to the hub. On a one-piece alloy wheel they are a single block of metal, which is why people use the words for the same thing.
What is the difference between a rim and a tyre?
The rim is metal and part of the wheel; the tyre is rubber and fits around it. They meet at the bead seat, where the tyre's bead sits.
Is the tyre part of the wheel?
Not in the standards. UN Regulation No. 124 defines the wheel as the member between the tyre and the axle. In everyday speech, "a wheel" often means the wheel with its tyre fitted.
Why do people call wheels rims?
On a one-piece wheel there is no separate rim, so the word drifted to the whole wheel, mostly for aftermarket wheels and mostly in American English.
Can I change my rims without changing my tyres?
You can move tyres to new wheels of the same rim diameter, if the new rim width is in the tyre's permitted range and the tyres are still sound. They have to be taken off and refitted by a tyre fitter, and the wheels balanced again.
What does 8J x 18 mean?
A rim 8 inches wide between the flanges, with a J-shaped flange, a one-piece rim (x), and 18 inches in diameter at the bead seat.
Is it tyre or tire?
The same word: tyre in British English, tire in American English.
Where to go next
For every part of a wheel, from the barrel to the valve hole, read parts of a wheel explained. For where a wheel sits in the arch, wheel offset; for how forged, cast and flow-formed wheels differ, forged vs cast vs flow-formed. Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards: see Onyx Forged and the catalogue.
Sources
- UN Regulation No. 124, replacement wheels: § 2.1, 2.1.3, 2.1.4, 2.4.4, 5.1.2.1 and Annex 3.
- ISO 3911:2021, wheels and rims for pneumatic tyres, vocabulary (free preview): 3.1, 3.2.1, 3.2.2, 3.7.1, 3.7.2, Figure 9.
- UN Regulation No. 30, pneumatic tyres (Official Journal L 201, 30 July 2008): § 2.4, 2.7, 2.8, 2.9, 2.14, 2.15, 2.18, 2.19, 2.27.
- 49 CFR 571.139, S3 definitions; 49 CFR 571.110, S4.4 (US).
- GOV.UK, MOT inspection manual, section 5, 5.2.2 and 5.2.3; Construction and Use Regulations 1986, regulation 27.
- General Motors, bulletin 08-03-10-006I, aluminium wheel bead seat corrosion.
- Continental, Technical Databook Car, 4x4, Van, permitted rims.
- A German wheel maker, Composition of a Wheel Rim; an American maker of forged wheels, two-piece wheel page.
- NIST Special Publication 811, Appendix B.9, conversion factors.
All pages were read on 15 October 2026.
The Onyx Forged design office.