Width, offset and ET: everything you need to know about wheel dimensions

Published on 22/07/2026

A wheel can have the right diameter and the right number of holes and still not fit correctly on your car. It may touch the brake calliper, rub inside the wheel arch or end up dangerously close to the suspension. The width and the offset, often indicated by the letters ET, largely determine its position. You also need to check the PCD, the centre bore, the load rating and compatibility with the tyre. Lots of figures, but a fairly simple logic once you take them in the right order. Here is how to read a wheel and accurately predict how it will fit.

How do you read a size such as 8.5Jx19 ET35?

A marking such as 8.5Jx19 ET35 sums up three main characteristics of the wheel. The number 8.5 corresponds to its inner width in inches. The letter J describes the profile of the flange on which the tyre bead sits. The number 19 indicates the nominal diameter of the wheel, also expressed in inches. Finally, ET35 corresponds to its offset in millimetres.

The 19-inch diameter is measured at the tyre bead seat, not at the outer edge of the wheel. A wheel described as 19 inches therefore looks slightly larger when you measure it from one edge to the other. This detail explains some surprising measurements taken with a simple tape measure.

You will also come across fuller markings such as 8.5Jx19 H2 5x112 ET35 CB66.6. H2 describes the shape of the inner humps that hold the tyre beads in place. 5x112 designates the PCD. CB66.6 indicates a centre bore of 66.6 mm. Each value answers a different question and none should be ignored.

A wheel’s width does not correspond to its outer width

The stated width is measured between the two tyre bead seats. An 8-inch wheel therefore measures 203.2 mm between these seats, since one inch equals 25.4 mm. Its flanges generally add around twenty millimetres to the overall width. The exact value varies according to how the wheel is built.

This distinction matters when you physically measure an old wheel. If you find about 225 mm from one outer edge to the other, it may very well be an 8-inch wheel and not a 9-inch one. To identify the size properly, first look for the cast, engraved or stamped marking on the back of a spoke.

Going from 8 to 9 inches adds exactly 25.4 mm to the nominal width. If the offset does not change, half of this difference goes inwards and the other half outwards. The new wheel therefore moves about 12.7 mm closer to the suspension while sticking out an extra 12.7 mm towards the wing.

The ET offset determines the position of the wheel

ET comes from the German Einpresstiefe. This value expresses the distance between the centre line of the wheel and its mounting face against the hub. It is given in millimetres. An ET45 wheel thus has a mounting face offset by 45 mm from its centre.

On the majority of modern cars, the offset is positive. The mounting face is on the outer side relative to the middle of the wheel. The higher the ET, the further the wheel sits inside the wheel arch. The lower the ET, the further it sticks out towards the wing.

Here is the rule to remember: a smaller ET pushes the wheel out, a larger ET brings it closer to the suspension. ET35 sticks out an extra 10 mm compared with ET45 if the width of the two wheels remains the same.

A zero offset, written ET0, places the mounting face exactly on the centre line of the wheel. A negative offset places it towards the inside. This configuration is mainly found on certain off-road vehicles, older cars or very specific set-ups.

Width and ET must always be calculated together

Comparing ET values alone often leads to the wrong conclusion. An 8-inch ET35 wheel and a 10-inch ET35 wheel obviously do not take up the same space. Their centres are in the same place relative to the hub, but the second extends an extra 25.4 mm on each side.

Take an original fitment of 7.5Jx18 ET45 and a new wheel of 8.5Jx19 ET35. The width increases by one inch, i.e. 25.4 mm. Half of that is 12.7 mm. The reduction in offset adds a further 10 mm towards the outside.

The new wheel will therefore stick out an extra 22.7 mm. On the inside, the increase in width makes it gain 12.7 mm, but the lower ET moves it 10 mm outwards. In the end, it will only come 2.7 mm closer to the suspension. A visually significant change, with a fairly small loss of inner clearance.

The formula for calculating the movement of the outer edge is as follows: half the increase in width, plus the difference between the old ET and the new ET. For the inner edge, take half the increase in width and add the difference between the new ET and the old ET.

A lower ET does not always give a better result

A wheel neatly aligned with the wing can transform a car’s appearance. However, lowering the ET drastically for the sole purpose of filling the wheel arch carries several risks. The tyre may touch the edge of the wing under compression, on full lock or when the car is loaded. On a lowered vehicle, a few millimetres are sometimes enough.

The position of the wheel also affects the geometry of the running gear. An offset very far from the original can change the scrub radius, the feedback through the steering wheel and the sensitivity to road irregularities. The load on the bearings also increases when the wheel is pushed far outwards.

The result may be twitchier steering, more kickback through the steering wheel or a tendency to follow ruts in the road. Not always dramatic on the first drive, but noticeable day to day. The right position is therefore not necessarily the one that puts the tyre within a millimetre of the wing edge.

Wheel spacers change the effective ET

A spacer sits between the hub and the wheel. It moves the whole wheel outwards. Its thickness is subtracted directly from the wheel’s offset. An ET45 wheel fitted with a 10 mm spacer therefore behaves like an ET35 wheel.

This calculation is simple, but fitting is not just a matter of choosing a thickness. You need to check the centring, the length of the bolts or studs, the contact area available on the hub and the compatibility of the system with the car. A poorly centred spacer can cause vibrations despite correct balancing.

The bolts must retain sufficient thread engagement without touching any component behind the hub. Their seat shape must also match the wheel. Finally, adding spacers may require documents and a vehicle inspection, depending on the vehicle and the set-up.

The PCD must match the hub exactly

The PCD is generally presented in the form 5x112. The first number indicates that the wheel has five fixing holes. The second corresponds to the diameter, in millimetres, of the imaginary circle passing through their centres.

A 5x112 wheel cannot be fitted directly to a 5x114.3 hub. The difference seems small, but the bolts then do not work along their axis. Forcing the fitting or relying on the bolts to centre the wheel is not acceptable.

Some vehicles exist with several PCDs depending on the generation, the engine or the chassis type. The commercial model is therefore not always enough. A Mercedes may use 5x112, but not all the associated sizes and centre bores are automatically compatible. The same caution applies to BMW, Audi, Volkswagen, Tesla and the Japanese manufacturers.

The centre bore ensures the wheel is centred

The centre bore is the diameter of the hole in the middle of the wheel. It sits on the hub’s spigot. When these two diameters match, the wheel centres itself correctly before tightening.

A wheel whose centre bore is too small cannot be fitted without machining, which is rarely desirable on a new wheel. If the bore is larger than the hub, a suitable centring ring can make up the difference. A 72.6 mm wheel can thus be centred on a 66.6 mm hub with the right ring.

The rings do not support the weight of the car once the bolts are correctly tightened. They position the wheel during tightening and prevent it from remaining slightly off-centre. A missing, damaged or wrongly sized ring often causes vibrations at certain speeds.

Wheel bolts are not all interchangeable

Two bolts with the same diameter and the same thread pitch can have different seats. The most common are conical and spherical. Some wheels use a flat seat with a washer. A conical bolt in a wheel designed for a spherical seat does not bear correctly.

You also need to check the length under the head, the strength class and the spanner size. Bolts that are too short lack thread engagement. Bolts that are too long may touch the handbrake components or protrude behind the hub.

Tightening must be carried out to the torque specified for the vehicle or the set-up, with clean contact surfaces. A torque wrench remains the right method. After removing wheels or fitting new ones, checking the tightness after the first few kilometres is a reasonable precaution.

Calliper clearance does not depend on ET alone

A wheel can have the right diameter, the right width and the right offset and still touch the brake calliper. The inner shape of the spokes and the depth of the wheel face are decisive. Two 8.5Jx19 ET35 wheels can offer very different clearances.

Large braking systems raise two questions above all. The inner diameter of the barrel must clear the calliper, and then the spokes must leave enough room in front of it. A very concave wheel does not necessarily offer more clearance. It all depends on its design.

This is why a calculator based solely on dimensions cannot guarantee every fitment. Reputable manufacturers validate their references by application or supply test templates for certain brakes. On a car fitted with a big brake kit, a physical measurement is often still necessary.

The tyre width must remain consistent with the wheel width

A tyre is approved for a range of wheel widths. A 225/40 R18 must not be fitted arbitrarily to just any 18-inch wheel. The nominal width of the tyre, its construction and its manufacturer’s specifications determine the authorised range.

On a narrow wheel, the sidewalls take on a more rounded shape. On a wide wheel, they become straighter, then start to stretch if you go beyond a reasonable configuration. A slight change in shape can alter steering precision and comfort. An excessively stretched fitment leaves the wheel rim more exposed and reduces the sidewall’s ability to absorb impacts.

The actual width of a tyre is not always exactly the stated width. Two tyres marked 245/35 R19 can differ by several millimetres depending on the brand, the model and the width of the measuring rim. A fitment calculated with only 2 mm of margin may therefore rub after a simple change of tyre.

Changing diameter means keeping an eye on the tyre circumference

Going from 18 to 19 or 20 inches does not mean that the complete wheel has to become much larger. The sidewall height is generally reduced to keep an outer diameter close to the original. This is the principle of the equivalent fitment.

An excessive variation in circumference affects the speedometer reading, the gear ratios, the operation of the electronic aids and the clearance in the wheel arch. On a four-wheel-drive vehicle, a difference between the front and rear axles can also put the drivetrain under strain.

Staggered fitments, with wider tyres at the rear, must therefore retain a consistent circumference between the two axles. It is not enough to choose two sizes that look visually matched.

A larger wheel also changes the way the car behaves

A large wheel often allows a tyre with a lower sidewall to be fitted. The steering may feel more direct and the car better controlled in changes of direction. On the other hand, the tyre absorbs joints, potholes and kerbs less well. Comfort decreases and the risk of damaging a wheel increases.

Weight deserves as much attention as diameter. A heavy wheel adds unsprung mass and rotational inertia. This can impair the work of the suspension, acceleration and braking. Switching to a larger but well-designed wheel sometimes gives a better result than a smaller, very heavy wheel.

The stated weight does not tell the whole story, however. Strength, maximum load and the distribution of material also count. A very light wheel that is unsuited to the weight of the vehicle is not a good choice.

Cast, flow formed or forged: what difference does the manufacturing make?

The majority of wheels on the market are cast. Aluminium is poured into a mould, then the part is machined and treated. This method makes it possible to produce a wide variety of models at a reasonable cost. The quality depends on the process, the design and the manufacturing controls.

Flow forming generally starts from a cast blank whose barrel is formed under pressure while rotating. This process can refine the structure of the material and reduce the amount of material needed in the barrel. The result is often a wheel lighter than a conventional cast model of comparable dimensions.

A forged wheel is formed from a block or blank subjected to high pressures, then extensively machined. It can combine strength and low weight, at a significantly higher price. The word forged still does not exempt you from checking the permissible load, the documents and the intended application.

The permissible load becomes critical on heavy vehicles

Every wheel has a maximum load. It must be sufficient to support the permitted load on the axle concerned. Simply dividing the car’s total weight by four is not the right method, because the distribution between the axles is not necessarily equal.

You need to refer to the axle loads specified for the vehicle and to the wheel manufacturer’s data. This point deserves particular attention on SUVs, vans, electric vehicles and plug-in hybrids, which are often heavier than combustion-engined models of comparable size.

The tyre also has a load index. The wheel and the tyre must each meet the needs of the vehicle. The capacity of one never compensates for a shortfall in the other.

Concavity cannot be chosen independently of the dimensions

Manufacturers sometimes offer several profiles for the same design: flat, concave or deep concave. The version available depends on the width, the diameter, the offset and the PCD. A 9.5J ET25 wheel can accept a deeper shape than an 8J ET50, because its geometry leaves more space for the spokes.

Ordering a wheel based solely on a photo can therefore lead to disappointment. The model fitted to a demonstration car in an 11-inch width will not necessarily have the same depth in an 8-inch size intended for a compact car.

The fitment must remain compatible with Swiss requirements

In Switzerland, mechanical compatibility is not always enough. Depending on the wheel, its dimensions and the difference from the original fitment, documents may be required for the vehicle inspection. In particular, the wheel and tyre must retain sufficient clearance, remain properly covered by the bodywork and not make contact in the various suspension and steering positions.

The situation varies according to the vehicle, the wheel reference and the documents available. A size listed in the vehicle data is easier to deal with than a heavily modified set-up with spacers, unusual tyres or a significant change in track width.

A generic homologation found online does not necessarily prove that your exact reference is covered in Switzerland. The wheel number, its dimensions, its load rating and the application must match. If in doubt, have the documents checked before ordering rather than at the time of the vehicle inspection.

Measurements on the car remain useful for a custom fitment

The original dimensions provide an excellent basis for calculation, but a modified car sometimes needs to be measured. Lowered suspension, different camber, a brake kit or reworked wings change the available margins.

Measurements should be taken in several situations: wheels straight, steering on lock, suspension compressed and vehicle loaded if necessary. You need to check the strut, the suspension arms, the hoses, the calliper, the wing edge and the wheel arch liner.

Do not calculate a fitment aiming for zero millimetres of margin. The tyre deforms, the suspension components move and manufacturing tolerances exist. The static position on level ground does not represent everything that happens on a bumpy road.

Here are the points to check before ordering

• The diameter and width of the wheel.
• The ET offset and the new position of the inner and outer edges.
• The PCD, for example 5x112 or 5x114.3.
• The centre bore and whether centring rings are needed.
• The type, thread and length of the bolts or nuts.
• Clearance for the callipers and suspension components.
• The tyre width authorised on the wheel.
• The rolling circumference and load index of the tyre.
• The maximum load supported by each wheel.
• The documents required for the vehicle and the Swiss vehicle inspection.

Open our wheel configurator

You now know the role of the width, the ET offset, the PCD and the centre bore. All that remains is to find the references designed for your car without manually comparing dozens of technical data sheets.

Open our wheel configurator, select your vehicle and discover the compatible models available at Rupteur.ch. You can then choose the diameter, style and finish, starting from a base suited to your car.

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