When purchasing new rims, many car enthusiasts are faced with mysterious markings, including the parameter ET raises the most questions. You can often hear the phrase that “a disc with a different offset will not fit” or, conversely, “it will fit, but will rub.” Understanding what exactly this parameter means is critical not only for aesthetics, but also for traffic safety, as well as the life of your car's suspension.
Disc offset is the distance between the vertical plane of symmetry of the wheel and the plane of application of the disc to the hub. This parameter is measured in millimeters and can be positive, negative or zero. Incorrect selection of value ET can lead to the wheel simply not going into place, touching the arch, or, even worse, creating a dangerous load on the chassis elements.
In this article, we will analyze in detail the physical meaning of offset, learn how to read markings on disks and understand what permissible deviations exist for different types of cars. You will learn how changing this parameter affects handling and why manufacturers strictly regulate tolerances.
Physical meaning and geometry of the ET parameter
To understand the essence of the parameter, it is necessary to visualize the geometry of the wheel. Imagine a disk lying flat. It has an inner part that presses against the hub and an outer part that faces the fender. Plane of symmetry is an imaginary line dividing the disk exactly in half along its width. The offset (ET) is the distance from this centerline to the mating plane.
If the mating plane is shifted outward relative to the center of the disk (closer to the front side), then the offset is considered positive. This is the most common situation for modern passenger cars. The greater the numerical value of the positive offset, the deeper the disc “goes” inside the arch when installed. In the labeling this is indicated as ET45 or Offset 45.
In the case when the mating plane is exactly in the middle of the disk width, the offset is considered zero (ET0). If the mount is shifted inward (closer to the brake calipers), then the offset becomes negative. Negative offset is typical for SUVs and wide wheels, where it is necessary to widen the track to increase off-road stability.
⚠️ Attention: Never focus only on the visual similarity of rims from different car models. Even if the bolt holes match, differences in offset may make installation impossible or dangerous.
It is important to note that car manufacturers indicate the recommended offset for a reason. Engineers calculate the kinematics of the suspension, the load on the wheel bearings and the position of the center of gravity specifically for the specific parameters of the disk. Changing wheel geometry changes roll-in shoulder, which directly affects the car’s behavior when cornering and braking.
Marking and decrypting disk parameters
You can find information about the disc ejection on its inner side. Typically the markings are stamped or cast into the metal at the back of the spokes. The standard entry looks like a set of numbers and letters, for example: 7.5J x 16 H2 5/112 ET 35 d66.6. Let's analyze this code element by element so that you can determine compatibility yourself.
The first number (in example 7.5) indicates the width of the rim in inches. Letter J indicates the type of rim flange (most common for passenger cars). The number 16 is the diameter of the rim in inches. Next comes the parameter PCD (5/112), which means the number of mounting holes and the diameter of the circle on which they are located. And only after this comes the desired parameter ET 35.
The last value (d66.6) is the diameter of the central hole (DIA or CO). This parameter is also important, since the disc must fit tightly onto the hub. If the disk center is larger than the hub, centering rings are used. If it’s less, the disc simply won’t fit. However, it is the departure ET determines how far the wheel will extend beyond the arch or go inward.
Is it possible to change the disc offset?
It is impossible to change the physical offset of a finished disk without compromising its integrity and strength. Attempts to weld spacers or grind off the mating plane lead to a shift in the center of gravity of the wheel and potential destruction of the disk under load. The only safe way to change the offset is to use disc spacers installed between the disc and the hub.
It is worth paying attention to the units of measurement. Unlike diameter and width, which are measured in inches, offset is always specified in millimeters. This is an international standard, so discs made in Germany, Japan or China will have the same designation. In some cases, instead of ET you can find the designation Offset or Einpresstiefe (German name), but the essence remains the same.
Effect of offset on chassis and handling
Many car owners strive to install wheels with a shorter offset so that the wheels visually “stick out” from the arches, giving the car a more aggressive look. However, this change in geometry has its consequences. Reducing overhang (for example, moving from ET40 on ET20) moves the wheel outward, which increases the vehicle track.
On the one hand, widening the track increases the lateral stability of the vehicle. The car becomes less prone to roll when cornering. However, on the other hand, this increases the load on the suspension elements. Wheel bearing begins to experience increased leverage, which can shorten its life. The load on the suspension arms and steering mechanism also increases.
As the offset increases (the wheel goes deeper into the arch), the situation changes. The wheel may begin to touch internal elements: shock absorbers, brake hoses or body elements during suspension travel. In addition, changes in reach affect the operation of stabilization systems (ESP) and anti-lock braking system (ABS), since the vector of force application changes during braking and acceleration.
Optimal wheel offset is a compromise between track width for stability and maintaining factory suspension loads for durability.
The change in reach is especially critical for cars with front-wheel drive and suspension type McPherson. Here, changing the roll shoulder can lead to unpredictable steering behavior, especially on slippery roads. The car may become more sensitive to ruts or, conversely, “sluggish” in reactions.
Permissible deviations and selection rules
Is there a universal formula for selecting disks? Unfortunately no. Each car has its own range of acceptable values. However, there are general rules that guide specialists. For passenger cars, deviation from the standard value within the limits is considered acceptable. ±5 mm.
If the standard offset of your car ET45, then installing the disk with ET40 or ET50, as a rule, will pass without serious consequences for the life of the chassis. However, this is only true if the width of the disk remains constant. If you change both width and offset, the calculations become more complicated as the inside of the disc may move toward the brake calipers.
For SUVs, tolerances may be wider, especially if the vehicle is used off-road. Here, negative offset is often used to install wide tires. But even in this case, it is necessary to check the clearances at maximum suspension travel and full steering rotation.
It is necessary to check the clearance between the inner part of the disc and the caliper, as well as between the outer part of the tire and the arch when the suspension is compressed. Sometimes installing the desired discs requires a suspension lift or trimming of the arches, which is already a serious intervention in the design.
Compatibility table and calculation examples
For clarity, let's look at how changing the rim width and offset affects the position of the wheel relative to the hub. Let's imagine that we are changing the parameters of the disk on a car. The table below shows the displacement of the internal and external boundaries of the disk relative to the standard position.
| Disk option | Width (inches) | Departure (ET) | Inward displacement (mm) | Outward displacement (mm) |
|---|---|---|---|---|
| Staff | 7.0 | 45 | 0 | 0 |
| Wider, same ET | 8.0 | 45 | +12.7 | +12.7 |
| Wider, smaller ET | 8.0 | 35 | +2.5 | +22.9 |
| Already, bigger ET | 6.5 | 50 | -6.3 | -19.0 |
| Negative ET | 8.0 | -10 | -41.9 | +86.4 |
The table shows that simply increasing the wheel width by 1 inch (25.4 mm) while maintaining the offset results in a wheel expansion of 12.7 mm in both directions. If we simultaneously reduce the offset (for example, from 45 to 35), then the wheel moves outward even more. That is why when selecting wide rims Often you have to reduce the offset so that the wheel does not touch the internal elements.
You can calculate the displacement yourself. 1 inch is equal to 25.4 mm. Half an inch is 12.7 mm. If you take a rim 1 inch wider, but with an offset of 10 mm less, then the wheel will come out to the outside by: 12.7 mm (half the width) + 10 mm (offset difference) = 22.7 mm. This is a significant change and is guaranteed to require an inspection of the arches.
⚠️ Attention: When calculating, keep in mind that the actual width of the tire may differ from that declared by the wheel manufacturer. The tire can be wider than the rim itself, and it is this tire that can rub against the arch, even if the rim itself fits the dimensions.
Practical advice for installation and testing
The process of installing discs with non-standard offset requires careful preparation. You should not rely only on the assurances of sellers or data from the Internet. Reality may differ due to the characteristics of a particular vehicle modification, year of manufacture, or the presence of non-standard elements (for example, large brakes).
Before purchasing a set of wheels, it is strongly recommended that you do a “fit-on”. Take one disk, install it on the hub and tighten the bolts. Without jacking up the car, turn the steering wheel all the way in both directions. Then, if possible, raise the car and check the clearances with the suspension fully extended (when it is extended) and compressed (when the car is lowered).
☑️ Check before purchasing discs
Particular attention should be paid to fastening elements. When installing discs with very different offsets, the standard bolts may be short (if the offset is reduced and the disc “sticks out”) or too long (if the disc is deep). Using bolts of the wrong length can cause them to rest against the end of the hub and the disk will not press, or they will dangle, causing runout.
Also remember the law. In some countries, installing wheels with parameters significantly different from the factory ones may lead to problems when passing a technical inspection or communicating with traffic police inspectors if the changes are regarded as interference with the design of the vehicle.
Use online rim calculators by entering the parameters of your car and the desired rims. They will automatically calculate the difference in offset and show how much the wheel will move relative to its normal position.
Frequently asked questions (FAQ)
Is it possible to install a disc with an offset of ET40 instead of the standard ET45?
Yes, a difference of 5 mm is considered acceptable for most passenger cars. The wheel will move outward by only 5 mm, which is visually almost imperceptible and will not create a critical load on the bearings. However, always check the arch clearance if you have wide tires.
What happens if the disc offset is greater than standard (for example, ET50 instead of ET35)?
The disc will go deeper into the arch. The main problem is that the inside of the wheel or the tire itself may begin to touch the suspension components, shock absorber or brake caliper. It can also impair the vehicle's stability on the road.
Does disc offset affect speedometer readings?
The offset (ET) parameter itself does not affect the wheel circumference, so it does not affect the speedometer readings. The speed and instrument readings are only affected by changes in the outer diameter of the tire. However, changing the reach changes the dynamics of acceleration and braking due to changes in the moment of inertia and the shoulder of force application.
How can I find out the standard disc offset for my car?
Information can be found in the vehicle's owner's manual (section "Wheels and Tires"), on a sign in the doorway (often indicating pressures and sizes), or simply by looking at the markings on the current factory wheel (ET or Offset designation). The data is also available in specialized catalogs for the car brand.
Is negative overhang dangerous for city driving?
For city driving, a negative offset is not optimal. It accelerates the wear of wheel bearings, makes the steering more sensitive to ruts, and also increases the risk of damaging the wheel on a curb. In addition, dirt from under the wheels will fly more strongly onto the car body.