An alloy wheel can look perfect in a photograph and still be wrong for your car. Two wheels with the same 18-inch diameter may sit in completely different positions, use different bolt patterns or fail to clear the brakes. This alloy wheel size guide focuses on the measurements that actually determine fitment: diameter, width, offset, PCD and centre bore. They must be considered together, not separately.
How to read the numbers on an alloy wheel
A specification might read 7.5Jx17 ET45, 5×112, CB 66.6. This is an illustrative example, not a recommendation for a particular vehicle. Here, 7.5 means rim width in inches; J identifies the flange profile; 17 is rim diameter in inches; ET45 means positive 45 mm offset; 5×112 describes the bolt pattern; and 66.6 mm is the centre bore.
These figures may be stamped or cast on the back of an alloy, although not all markings appear together. Check the wheel manufacturer’s full specification and your vehicle’s approved fitments before ordering.
Wheel diameter: what 17 or 18 inches means
Diameter is measured across the tyre bead seats, not the outer edges of the wheel. A 17-inch alloy needs a tyre designed for a 17-inch rim. In a tyre code such as 225/45 R17, the final 17 identifies rim diameter; R identifies radial construction.
Moving from a 17-inch to an 18-inch wheel does not automatically make the complete tyre-and-wheel assembly an inch taller. A lower-profile tyre can sometimes keep the overall rolling diameter close to the original. However, tyre load rating, speed rating, rim-width compatibility and vehicle approval still matter. Larger rims may also mean less sidewall cushioning over rough roads.
Start with the manufacturer’s approved sizes in your handbook or vehicle information label. Our tyre size markings explained guide can help you read the accompanying tyre dimensions.
Alloy wheel width: more than an appearance choice
Alloy wheel width is measured between the inner faces of the tyre bead seats, rather than across the outer rim lips. A 7.5J alloy is nominally 7.5 inches wide at that point. Wider rims may accept wider tyres, but there is no universal tyre-to-rim combination.
Check the tyre manufacturer’s permitted rim-width range and your vehicle’s approved combinations. Stretching a tyre over an unsuitable rim or squeezing it onto one that is too narrow can compromise its intended shape and performance. Remember that a fitted tyre’s sidewalls can extend beyond the rim, changing the available clearance.
Wheel offset explained: why ET matters
Offset measures the distance in millimetres between a wheel’s centreline and its hub-mounting surface. ET comes from the German term Einpresstiefe. With positive offset, the mounting surface is towards the outer side of the centreline. At zero offset, the two align. With negative offset, the mounting surface is towards the inner side.
On wheels of identical width, reducing offset from ET45 to ET35 moves the wheel 10 mm outwards. Raising it from ET35 to ET45 moves the wheel 10 mm inwards. Moving out can reduce arch clearance; moving in can reduce suspension clearance. Neither direction is inherently safer.
A real-world width-and-offset example
Suppose you replace a 7J ET45 wheel with an 8J ET35 wheel. The new rim is one inch, or 25.4 mm, wider, adding 12.7 mm to each side of its centreline. Reducing offset by 10 mm moves that centreline outwards too. As a result, the nominal outer rim edge projects 22.7 mm farther out, while the inner rim edge sits about 2.7 mm closer to the suspension.
This calculation shows why offset cannot be judged alone. It describes rim geometry, not actual tyre, brake or bodywork clearance. Our wheel offset calculator guide explains these comparisons, but physical checks are still necessary.
Wheel PCD: the bolt pattern must match
Wheel PCD means pitch circle diameter. The first figure gives the number of wheel bolt or stud holes; the second gives the diameter in millimetres of the imaginary circle through their centres. A 5×112 wheel has five holes on a 112 mm circle. It cannot simply be substituted for 5×114.3 because the figures look similar.
Verify the PCD for your exact vehicle model and variant. Check the bolt or nut seat shape, thread, suitable length and tightening specification as well. Matching the wheel PCD alone does not make the fixings compatible.
Centre bore size: locating the wheel on the hub
The centre bore is the hole in the middle of the wheel that fits over the vehicle’s hub locating lip. A bore smaller than the hub prevents proper seating. Some aftermarket alloys use a larger bore with correctly specified hub-centric rings to match the vehicle hub.
Those rings help centre the wheel during fitting; they cannot correct an incorrect PCD, unsuitable fixings or insufficient wheel load capacity. The mating surfaces must sit cleanly and flush. If the required centre bore size or ring is uncertain, have a wheel specialist confirm the combination.
Other fitment checks that numbers cannot settle
Wheel dimensions provide a technical starting point, not a guarantee. Brake clearance depends partly on spoke shape and barrel design. Two rims with identical diameter, width and offset may clear the same caliper differently. Tyre clearance can also change as the suspension moves or the steering reaches full lock.
Confirm the alloy’s load rating, appropriate tyre specifications, fixing hardware and any tyre pressure monitoring equipment. Have the wheels professionally fitted and balanced, using the manufacturer’s tightening procedure. Check that the tyres remain clear of bodywork and suspension through their normal operating range.
For UK drivers, the DVSA MOT inspection manual lists incompatible wheel fixings and tyres fouling components as defects. An MOT pass is not a universal endorsement of aftermarket fitment. Disclose non-standard wheel changes to your insurer and check the implications for cover. Our wheel fitment checklist can help before you buy.
Frequently asked questions
Can I fit wider alloys without increasing diameter?
Potentially, yes. Width and diameter are independent. The wider wheel must accept an appropriate tyre and still leave enough room for brakes, suspension and bodywork.
Does lowering ET always move a wheel outwards?
Yes, when the width stays the same. If width also changes, calculate both effects to establish where the rim edges will sit.
Can hub-centric rings fix an incorrect centre bore?
Correctly specified rings can adapt certain oversized wheel bores to a smaller hub locating lip. They cannot make an undersized bore fit or correct the wrong bolt pattern.
Is matching the PCD enough for safe fitment?
No. Diameter, width, offset, centre bore, fixings, wheel load rating, tyre compatibility and available clearance must also be checked.
Make the measurements work together
The sensible order is simple: find the vehicle’s approved fitments, compare every dimension of the proposed wheel and confirm physical clearance before paying for a new set. A few millimetres can change where an alloy sits. Choosing by measurements first gives you far more confidence when you finally choose the design.