Wheel offset (ET) explained
- Module: Dimensions
- Level: Intermediate
- Reading time: 11 min
The offset, marked ET on a wheel, is the distance in millimetres between the wheel's mounting face, the one that presses against the hub, and the plane that cuts the wheel in half across its width. With a positive offset (ET45, for example), the mounting face is on the outer side and the wheel sits further in under the wing; with a negative offset, it is on the inner side and the wheel sits further out. The rule that serves every day: at equal width, the smaller the ET, the further out the wheel sits, millimetre for millimetre.
The offset cannot be seen in a photo. Yet it decides whether a wheel looks sunk into the car or sits right at the edge of the wing, whether it rubs or not, and part of what you feel through the steering wheel. Here is how it is defined, measured and calculated, what it changes, and why so many pages get the direction wrong.
What does ET mean on a wheel?
ET comes from the German Einpresstiefe, the “press-in depth”. In English it is called the offset, in French déport. The UN regulation on replacement wheels (No. 124), adopted into European law, defines it as the distance between the mounting face of the wheel disc and the centre line of the rim, and requires it to be marked on the wheel, in millimetres.
Two reference points are enough to picture it:
- the centre plane: an imaginary plane that cuts the wheel into two halves of the same width, between its two flanges;
- the mounting face: the flat surface, at the back of the centre, that presses against the car's hub.
The offset is the gap between the two. The hub, for its part, does not move: it is the wheel that positions itself around it. Moving the mounting face within the wheel therefore moves the whole wheel within the wheel arch. On a wheel marked 8.5Jx19 ET45, the mounting face is 45 mm from the centre plane, on the outer side.
Positive, zero or negative offset: how to tell them apart?
| Offset | Where the mounting face is | What you see |
|---|---|---|
| Positive (ET > 0) | Between the centre plane and the outer side | The wheel wraps around the hub and sits further in under the wing |
| Zero (ET 0) | Exactly on the centre plane | The wheel is centred on its mounting face |
| Negative (ET < 0) | Between the centre plane and the inner side | The centre is deeply dished, the wheel sits clearly further out |
The original wheels of the recent cars we have recorded from manufacturer documents all have a positive offset: ET19 on a 22-inch Audi winter wheel for the RS 6 Avant, ET34 at the front and ET45 at the rear on the 20-inch wheels of the Tesla Model 3, ET52 at the front and ET67 at the rear on the 19 and 20-inch wheels of the Porsche 911 Carrera (992).
The classic trap, found on several pages at the top of Google: getting the direction the wrong way round. A larger ET brings the wheel in, a smaller ET pushes it out. And the ET is not an absolute “sink-in” depth: it is counted from the middle of the wheel.
Where do you find a wheel's offset?
It is marked on the wheel, in millimetres, most often on the back of a spoke or in the mounting area, in the form “ET45”. For standard wheels, the owner's manual or the manufacturer gives it. For a wheel without a marking, it has to be measured.
How is a wheel's offset measured?
On a removed wheel, without its tyre, you need two measurements and a perfectly straight rule:
- The overall width: from the outer edge of one flange to the other. It exceeds the marked width, taken between the flanges, by about 25 mm.
- The back distance: lay the rule across the back of the wheel, on the inner flange, and measure to the mounting face. This is what Americans call backspacing.
- The calculation: offset = back distance − (overall width ÷ 2).
Example: 240 mm overall and 165 mm of back distance. The centre plane is 120 mm from the inner edge; the mounting face is 45 mm further towards the outside. Offset: 165 − 120 = ET45. If the result is negative, so is the offset.
How do you calculate what changes when the offset or the width is changed?
Two geometric rules are enough, counted from the mounting face, which does not move:
- on the outer side, the edge of the wheel is at (width ÷ 2) − ET;
- on the inner side, it is at (width ÷ 2) + ET.
The width is taken in millimetres (1 inch = 25.4 mm: an 8.5-inch wheel measures 215.9 mm between its flanges). These positions are taken at the inner edge of the flanges, where the width is measured; the differences between two wheels, on the other hand, are exact. The track width, in the table, is the distance between the left and right wheels of the same axle. Three consequences follow:
| Change | Outer side | Inner side | Track width (per axle) |
|---|---|---|---|
| ET reduced by 10 mm, same width | the wheel moves out by 10 mm | it moves 10 mm away from the suspension | + 20 mm |
| ET increased by 10 mm, same width | the wheel moves in by 10 mm | it moves 10 mm closer to the suspension | − 20 mm |
| Width + 1 inch, same ET | the wheel moves out by 12.7 mm | it moves 12.7 mm closer to the suspension | unchanged |
Example 1, two 19-inch wheels. Compare an 8Jx19 ET45 with a 9Jx19 ET35:
- outer side: 101.6 − 45 = 56.6 mm, compared with 114.3 − 35 = 79.3 mm. The wheel moves out by 22.7 mm;
- inner side: 101.6 + 45 = 146.6 mm, compared with 114.3 + 35 = 149.3 mm. It moves closer to the suspension by 2.7 mm;
- the centre plane moves by the difference in ET, 10 mm towards the outside: the track width grows by 20 mm on the axle.
Example 2, a real case. The workshop manual of the Tesla Model 3 (2024 onwards) lists, in the 20-inch size, a 9-inch ET34 wheel at the front and a 10-inch ET45 wheel at the rear. On the outside, 114.3 − 34 = 80.3 mm at the front, 127 − 45 = 82 mm at the rear: relative to its hub, each wheel sticks out by almost the same amount. On the inside, 148.3 mm at the front, 172 mm at the rear: the extra inch of width has gone almost entirely inwards. That is the role of the offset: placing each wheel where the car has room.
The offset calculator does these sums for any two wheels and shows the difference on each side. It does not say whether the wheel will fit: only the measurements of your car can say that.
Does the offset change the way the car drives?
Yes, because it moves the point where the tyre touches the road relative to the axis around which the wheel pivots when you steer. The distance between the two, at ground level, is called the scrub radius (in French déport au sol): the same French word, but a different thing from the ET. The manufacturer sets it along with the whole suspension geometry.
Reducing the ET moves the wheel out and shifts this scrub radius towards positive by the same amount. As it moves away from zero, chassis handbooks describe the effects:
- heavier steering: the larger the scrub radius, the more effort it takes to steer;
- more kickback through the steering wheel, on a bumpy road or under braking when one wheel grips better than the other; recent cars aim instead for a small, or even negative, scrub radius, to stay stable in that situation;
- a longer lever arm on the wheel bearing, which puts more load on one of its rows.
The offset also enters the calculation of the bending test that Regulation No. 124 requires for replacement wheels. Within a few millimetres, you feel nothing; over several centimetres, the car changes character.
Why does a wheel rub?
A wheel has two neighbours, and the offset always brings it closer to one while moving it away from the other:
- on the outside, the wing: when the suspension compresses, in a loaded corner or over a bump, and when you steer, the tyre rises towards the edge of the wing;
- on the inside, the suspension, the steering and the brakes: the damper, the arms, the ball joints, and the brake caliper, housed inside the wheel.
To assess a replacement wheel, Regulation No. 124 generally sets a minimum clearance of 3 mm from the brakes, 4 mm from the suspension and steering, and 2 mm for the balance weights, in the least favourable case. These are reference points for assessing a replacement wheel under the regulation, not a fitting rule, but they give the order of magnitude: a few millimetres.
For the caliper, the offset does not tell the whole story. Two wheels of the same diameter and the same ET may or may not clear a large caliper depending on the profile of their spokes, on the inner side. That is a separate measurement, and the reason why a design office asks for the exact version of the car and of its brakes. The tyre matters too: on the same wheel, a wider tyre, or one with a more rounded sidewall, sticks out further towards the wing.
What offset tolerance applies?
No regulation sets an offset tolerance valid for all cars. The “±5 mm” often quoted does not refer to any cited standard. What counts are the limits of the car itself: nothing touches, with the wheels on full lock as well as with the suspension compressed, and the wheel stays covered by the wing. The inspection rules give these limits:
- in France, the technical instruction for inspectors classes as a major defect a wheel that protrudes beyond the bodywork or touches another component, a tyre that rubs or may rub against a rigid component such as a wing, and treads that are not sufficiently covered;
- in Belgium, fitting wheels or tyres other than the original ones may not widen the track by more than 2% (4% for an off-road vehicle), according to the inspection body Autosécurité.
A major defect requires a re-inspection. Our page on the roadworthiness test for forged wheels goes through these points country by country, with their sources.
The most frequent mistakes
- Reading the offset the wrong way round. Large ET = wheel further in; small ET = wheel further out.
- Comparing ETs without looking at the width. A wider wheel at the same ET sticks out on both sides, by half the difference in width.
- Forgetting spacers. A spacer placed between the hub and the wheel acts like an ET reduced by its thickness: a 10 mm spacer on an ET45 wheel positions it like an ET35.
- Copying the offset of another car. The same PCD does not mean the same offset: the hub, the track width, the brakes and the wings differ from one model to another, sometimes from one version to another.
- Forgetting the rear. As on the Model 3 above, the front and the rear do not always have the same width or the same offset.
How the offset is decided at Onyx
We do not ask you to choose an ET, and we do not give you one in passing during a conversation: an offset figure only means something in relation to a specific car, its brakes, its ride height and its tyres.
The Onyx approach — The offset is at the heart of every Onyx Forged made-to-measure project. The wheel position you choose (“As on the original wheels”, “Further out” or “Flush with the wheel arch”) is a wish about the look: the Onyx Forged design office turns it into an offset, and where needed a width, calculated for your car as you described it (brakes, suspension, bodywork), then submits them to you in writing before machining. Other tyres or a different ride height can change the position obtained.
Frequently asked questions
ET35 or ET45: which sits further out?
The ET35. At equal width, it places the wheel 10 mm further out than the ET45, and widens the track by 20 mm on the axle.
Can a wheel's offset be changed?
Not on an existing wheel: it is set by the machining of the centre, or by the assembly of a wheel made of several pieces. A spacer moves the wheel, without changing the wheel itself. For a different offset, you need a different wheel.
Does a spacer change the offset?
It moves the wheel exactly as an offset smaller by its thickness would, and lengthens the lever arm on the hub. Onyx Forged wheels are designed to be fitted without spacers.
Can you fit a different offset from the original?
Yes, within the limits of what the car accepts: no contact with the wing, the suspension or the brakes, with the wheels on full lock as well as with the suspension compressed, and a wheel that stays covered by the wing. It is a calculation specific to each car.
In the next lesson, we finish the module on dimensions with the PCD and the centre bore: how the wheel is fixed to your car.
Lesson written by the Onyx Forged design office.
This lesson is part of the Onyx Academy, the free course on custom forged wheels.