In short. Choosing aftermarket wheels is seven checks, and the car decides most of them. The bolt pattern (PCD) and the centre bore come from the hub and are not a matter of taste. The diameter is a choice, but the tyre that goes with it must keep the overall rolling diameter close to the original. The width must fall in the range of rims the tyre is made for, and the offset (ET) then decides where both edges of the rim end up against the suspension and the wing: changing either moves the edges, and the arithmetic is simple. The seat of the bolts or nuts must match the wheel. The load rating of one wheel must be at least half the maximum load of the more heavily loaded axle, which is printed on a plate on the car. Weight comes last: compare one wheel against another at the same size, never a percentage. Whatever you choose, read the numbers from your car's documents, check them against the wheel's specification, and do not take a size or an offset from a forum post for a different version of your car.
In which order should you check the numbers?
Start with what the car imposes, then decide what is yours to choose. A wheel that fails one of the first checks is not a candidate, however good it looks.
| Check | Written as | Who decides | Where to find it |
|---|---|---|---|
| 1. Bolt pattern and centre bore | 5x112; the bore in mm | The hub | Handbook, maker's wheel list, marked inside the original wheel |
| 2. Diameter | 19 (inches) | You, within what the car allows | The car maker's list of wheel and tyre sizes for your version |
| 3. Width | 8.5J | You, within the tyre's range | Tyre maker's data (permitted rims) |
| 4. Offset | ET45 | The car, through clearances | Original wheel, maker's list, a fitter's measurement |
| 5. Seat and fixings | Cone 60°, ball, flat | The wheel | Wheel maker's fitting instructions |
| 6. Load rating | kg per wheel | The car's axle loads | Manufacturer's plate, then the wheel's specification |
| 7. Weight | kg per wheel | You | Wheel maker's specification, or a scale |
A marking such as 7,0 J x 17, ET49, LK112/5, which Volkswagen uses to explain its own wheels, packs four of these into one line: width 7 inches, a J flange, diameter 17 inches, offset 49 mm and a pattern of five bolts on a 112 mm circle (Volkswagen). Our guide to what size wheels fit my car explains how to read it and where to find it for your exact version.
Check 1: does the wheel match the hub?
The bolt pattern, or PCD, is the number of bolts and the diameter of the circle they sit on. It must be identical: 5x112 and 5x114.3 are different patterns, and a wheel that "nearly" fits is forced onto its bolts off centre. The centre bore is the hole in the middle of the wheel. It must be equal to the hub's spigot, or larger with a centring ring that fills the gap. Our guide to PCD, bolt pattern and centre bore explains how to read and measure both; we also have pages on the 5x112 and 5x120 patterns.
There is nothing to choose here. If the pattern differs, the only ways round it are an adaptor, which adds a joint, or a wheel drilled for your car. Our guide to wheel spacers and adaptors covers the first.
Check 2: what diameter, and what does it change?
The diameter is the size of the wheel at the bead seats, in inches; one inch is exactly 25.4 mm (NIST SP 811). It is the number people choose first, and the one that drags the others with it.
A larger wheel does not make a larger wheel and tyre. To keep the same overall diameter, the tyre's sidewall gets lower as the wheel gets larger. Two reasons to stay close to the original overall diameter:
- The speedometer. UN Regulation No. 39 requires that the speed shown is never less than the true speed (§ 5.3; Regulation No. 39). A tyre with a larger overall diameter makes the car go faster than it reads.
- Room in the arch. A taller wheel and tyre is closer to everything around it.
The smallest diameter you can fit is set by the brakes: a wheel must clear the caliper, and calipers differ from one version of a car to another. The largest is set by the car maker's list of sizes for your version, by the tyres that exist in that size, and by how much sidewall you want. Our tyre size calculator compares the overall diameter of two sizes; our guide to reading a tyre size explains the markings.
Check 3: how wide can the wheel be?
Width is measured between the flanges, the lips that hold the tyre, in inches. It is not chosen alone: a tyre is made for a range of rim widths, and the wheel must fall inside that range.
For each tyre size, the European Tyre and Rim Technical Organisation (ETRTO) sets a measuring rim and a range of permitted rims, and tyre makers publish them. Continental's technical data book, based on ETRTO values, also shows how the fitted tyre grows with the rim: about 5 mm for each extra half inch, for example 234, 239 and 244 mm for a 225/45 R17 XL on rims of 7½, 8 and 8½ inches (Continental).
So the order is: choose the tyre size, read its permitted rims in the tyre maker's data, then choose a width inside that range. A width at either end of the range changes the shape of the sidewall; ask whoever fits your tyres. A rule of thumb that turns a tyre width into a rim width by a percentage is no substitute for the table.
Check 4: where will the edges of the rim end up?
Offset (ET) is the distance between the wheel's mounting face and the centre line of its rim. A higher offset pulls the wheel into the car, a lower one pushes it out; our guide to wheel offset explains positive, zero and negative offsets.
Width and offset have to be read together, because what touches is an edge. Measured from the mounting face:
- Inner edge (towards the suspension) = half the width + ET
- Outer edge (towards the wing) = half the width − ET
A wider rim grows by half its extra width on each side; a change of offset moves both edges the same way by the same amount. Here is the arithmetic for a reference wheel of 8.0J ET45. It is an example to show the method, not a recommendation for any car.
| Wheel | Inner edge moves towards the suspension by | Outer edge moves towards the wing by |
|---|---|---|
| 8.0J ET45 (reference) | 0 | 0 |
| 8.5J ET45 | 6.4 mm | 6.4 mm |
| 9.0J ET45 | 12.7 mm | 12.7 mm |
| 9.0J ET35 | 2.7 mm | 22.7 mm |
| 9.0J ET52 | 19.7 mm | 5.7 mm |
| 8.0J ET35 | −10 mm (moves away) | 10 mm |
Two readings of the table. A wider wheel cannot keep both edges where they were: it takes room on one side, the other or both. And the rim is not what touches first: the tyre bulges past the flange, and its width depends on the rim, as check 3 shows.
How much room is enough? When UN Regulation No. 124 assesses a replacement wheel whose inner contour is tighter than the car maker's own, it asks as a rule for at least 3 mm between the wheel and the brake components, in the worst case (for example with new brake linings), 4 mm to the suspension components and 4 mm to the steering components; the check may be made statically or dynamically (Annex 10, § 2.1; Regulation No. 124). Those are assessment criteria, not fitting advice; they show how small the margins are. In our reading, a check on the car standing still says nothing about full steering lock or a compressed suspension.
The wheel offset calculator does this arithmetic for any two wheels. It compares; it does not know your car. The numbers that tell you whether a wheel clears are your own measurements, or the clearances the wheel's designer checked for your version. If you measure with a wheel off, the car must be raised and resting on axle stands, never on the jack alone.
Check 5: do the bolts or nuts suit the wheel?
The part of the bolt or nut that presses on the wheel is its seat: a 60° cone, a radius (ball) or a flat seat with a washer. It must match the hole in the wheel. UN Regulation No. 124 asks that the profile of the fixings be compatible with the profile of the location hole, and that the length of thread engagement be the same as with the manufacturer's own fixings (Annex 10, § 2.3). Original bolts can be wrong for an aftermarket wheel, if the seat differs or the wheel is thicker at the hub; the wheel maker states which fixings to use.
The same regulation asks the wheel maker to give the tightening torque, to recommend a calibrated torque wrench, and to say that the fixings must be tightened again after 50 km (Annex 10, § 1.4). The torque figure itself comes from the car maker or the wheel maker, never from a general table; our guide to wheel nut torque covers the method.
Check 6: how much load must one wheel carry?
The load rating is the load one wheel is made to carry. A wheel maker's buying guide defines it as the weight a single wheel can statically hold (Konig). It is printed in the wheel's specification and often stamped on the wheel. The figure it must reach comes from the car, not from its weight on a scale.
The method. Find the maximum permissible load on each axle. Take the larger of the two and divide it by two: that is the most one wheel on that axle can be asked to carry. The wheel's load rating must be at least that.
Where the axle loads are. In Europe, on the manufacturer's statutory plate fixed to the car (the handbook says where): it gives the maximum laden mass and the maximum mass on each axle (Implementing Regulation (EU) 2021/535, Annex II). In the United States, on the label the manufacturer fixes to the driver's door pillar or door edge, as the gross axle weight rating (GAWR) for each axle (49 CFR § 567.4).
Why not a quarter of the car's weight? Because the load is not shared equally, and because the plate gives the car loaded, not empty. BMW publishes the figures for the XM (BMW PressClub):
| BMW XM, published data | Value |
|---|---|
| Unladen weight, as published | 2,710 kg |
| A quarter of it | 677.5 kg |
| Maximum load, rear axle | 1,825 kg |
| Half of it: the most one rear wheel carries | 912.5 kg |
| Original rear tyre | 315/40 R21 115 Y XL |
| Load index 115, in kilograms | 1,215 kg |
The quarter of the empty weight underestimates the rear wheel by 235 kg: the most a rear wheel can be asked to carry is 35 per cent more than that quarter. The tyre's load index converts to kilograms through a table in UN Regulation No. 30 (Annex IV; Regulation No. 30): 103 is 875 kg, 108 is 1,000 kg, 110 is 1,060 kg and 115 is 1,215 kg.
How the tyre and the wheel relate. The Japanese JWL standard designs a wheel from the tyre: its basic load F is the load of the tyre with the highest load capacity among those specified for the wheel, and its radial load test runs at 2.25 times that load (§§ 2.1.1 and 2.2.1; MLIT, annex 2). In practice, check both: the tyre's load index against the axle, and the wheel's load rating against the axle. Heavy cars, SUVs and electric cars are where a wheel made for a light car falls short.
Check 7: how much does the wheel weigh?
Weight is the one number that is entirely yours to judge. A lighter wheel reduces unsprung and rotating mass. Porsche and Manthey quote a figure for their forged wheel set for the 911 GT3 (992.2): unsprung and rotating masses reduced by six kilograms against the standard wheels (Manthey Racing).
Compare like with like: one wheel against another, at the same diameter and width. A percentage without the two wheels behind it means nothing, and a well-designed cast wheel can weigh less than a heavy forged one. Our guides to lightweight wheels and unsprung weight and to forged, cast and flow-formed wheels explain what the weight changes and why the process alone does not decide it.
What should you ask the seller?
| Ask for | Why |
|---|---|
| The full designation: width, diameter, offset, PCD, centre bore | Each must be checked against your car, check by check |
| The load rating of one wheel, in kilograms | It must be at least half the heavier axle's maximum load |
| The seat and the fixings to use | Wrong seats and short bolts are the usual fitting faults |
| The weight of one wheel at your size | The only fair comparison |
| The test standard and who applied the mark | JWL is applied by the maker on its own responsibility; VIA after a third-party body confirms it (NAPAC) |
| Whether the wheel was checked for your exact version | Brakes and arches differ inside one model range |
After the purchase: tyres, sensors, MOT and insurance
- Tyre pressure sensors. If your car has direct sensors, the new wheels need a valve hole that takes them. Our guide to TPMS and the MOT explains what is checked.
- MOT. In Great Britain the inspection manual lists a wheel and its fixings that are not compatible as a major defect, checks the tyre's load and speed rating, and fails a tyre fouling the car at the time of the test (section 5.2; MOT inspection manual).
- Insurance. The Financial Ombudsman Service gives changing alloy wheels as an example of a modification and advises telling the insurer (FOS).
Outside Great Britain the rules differ; look up the ones where your car is registered.
Common mistakes
- Choosing the diameter first and the tyre last. The tyre's overall diameter and its permitted rims decide whether the diameter works.
- Copying an offset from another car. The same model can have other brakes, another track or other arches in another version.
- Changing width without recalculating offset. A wider wheel at the same ET moves both edges by half the extra width.
- Using a quarter of the empty weight as the load per wheel. Use half the heavier axle's maximum load.
- Reusing the original bolts on a wheel with another seat or another thickness at the hub.
- Comparing weights given as percentages. Ask for kilograms, wheel against wheel.
Frequently asked questions
What should I check first when buying aftermarket wheels?
The bolt pattern and the centre bore. If they do not match the hub, nothing else matters.
How do I work out the load rating I need?
Take the larger of the two maximum axle loads on the car's plate or door label and divide by two. The wheel's rating must be at least that.
Can I fit a wider wheel with the same offset?
The wheel will be wider on both sides, by half the extra width each side. Whether it clears depends on your car's margins; check with the offset calculator and your own measurements.
Does a bigger wheel change the speedometer?
Only if the overall diameter of wheel and tyre changes. Keep it close to the original by choosing a tyre with a lower sidewall.
Is a lighter wheel always better?
Lighter at the same size and the same load rating, yes in principle. A wheel that is light because it is rated for a smaller load is not a fair comparison.
Do aftermarket wheels need new wheel bolts?
Often. The seat and the length of the fixings must suit the new wheel; the wheel maker states which ones.
Can I use the sizes from a forum or a friend's car?
Use them as a starting point at most. Only your car's documents and measurements tell you what fits your version.
Where to go next
For each check in detail: what size wheels fit my car, PCD and centre bore, wheel offset, reading a tyre size, lightweight wheels and forged, cast and flow-formed wheels.
When you order forged wheels from Onyx Forged, a French brand, you are not asked to work out the offset, the bolt pattern, the width or the load rating yourself: they follow from your car. The Onyx Forged design office compares them with the car maker's documents for your exact version, designs the wheels to JWL/VIA standards, and goes through the figures with you before anything is made. Onyx Forged does not recommend a size or an offset: a figure only means something once it has been checked against the car maker's documents for your car. See Onyx Forged and the catalogue.
Sources
- UN Regulation No. 124, replacement wheels: Annex 10 §§ 1.4, 2.1 and 2.3.
- Continental, Technical Databook 2015-2016: permitted rims and fitted tyre widths, based on ETRTO values.
- UN Regulation No. 30, Annex IV; UN Regulation No. 39, § 5.3.
- BMW PressClub, BMW XM technical data (09/2022).
- Implementing Regulation (EU) 2021/535, Annex II; 49 CFR § 567.4.
- Konig, choosing aftermarket wheels.
- MLIT, technical standard for light-alloy disc wheels (JWL); NAPAC, JWL and VIA.
- Manthey Racing, lightweight wheel set for the 911 GT3.
- Volkswagen, wheel marking; NIST SP 811, appendix B.9.
- GOV.UK, MOT inspection manual, section 5; Financial Ombudsman Service, car modifications and insurance.
The BMW data, Implementing Regulation (EU) 2021/535 and the Japanese JWL standard were read on 4 October 2026; all other pages on 15 October 2026.
The Onyx Forged design office.