When selecting new rims for a car, many owners are faced with the mysterious abbreviation ET, which plays a critical role in suspension geometry. Incorrectly selected disc offset can lead to friction between the tire and body elements, accelerated wear of wheel bearings, and even loss of vehicle controllability. That is why knowing where to look for this parameter is a basic skill for any motorist planning to replace the “shoes” for his car.
Most often, the numbers you are looking for are stamped on the inside of the wheel itself, but getting to them is not always easy, especially if the wheels are already installed on the car. In some cases, markings can be erased by time, corrosion or a layer of dirt, which forces you to look for alternative ways to obtain information. Understanding the physics of the process will help you not only find the number, but also correctly assess whether a disc with a different offset value will suit your car without fatal consequences.
In this article, we will analyze in detail all the possible locations of the markings, learn how to read codes on stamped and cast disks, and also consider methods for calculating the parameter if a visual inspection does not produce results. Data Accuracy this is most important here, since an error of even a few millimeters can significantly change the behavior of the car on the road.
Physical markings on the inside of the disc
The most reliable and direct way to find out the offset is to inspect the inside of the wheel rim. It is there that the manufacturer is obliged to apply all technical information in accordance with international standards. You will have to remove the wheel from the car, since it is almost impossible to see this area through the spokes of cast wheels or technological holes of stampings without dismantling it.
Look for the inscription, which usually looks like ET 35, ET45 or Offset 0. The number after the letters indicates the distance in millimeters from the mating plane of the disk to its vertical axis of symmetry. On stamped discs this information is often embossed in relief on the inside of the spokes or on the rim. On diecast models the data can be painted or laser engraved on the back of one of the spokes.
⚠️ Attention: If you see the inscription ET 0 or a negative value (for example, ET -15), this means that the disk has a specific geometry typical for SUVs or sports cars. Installing such wheels on a civilian vehicle without modifying the arches will result in the tire flying out beyond the dimensions of the body.
Often markings are applied with durable paint, which over time can fade or become clogged with bitumen. Before searching, it is recommended to thoroughly clean the inner surface of the disk with a brush and dehydrator. Sometimes manufacturers hide the markings under a layer of protective varnish or even under a decorative cap, which must be carefully removed.
- 🔍 Look for the combination of letters ET or OFFSET next to the number.
- 🧼 Thoroughly clean the inner surface of the disc from dirt and grease.
- 📏 Check all the spokes; if the disc is cast, the markings can be on any of them.
- ⚙️ Pay attention to other parameters nearby: diameter, width and bolt pattern.
It is important to understand that mating plane - this is the surface with which the disc adheres to the car hub. All measurements are taken from it. If you cannot find the ET marking, check to see if the disc is a low quality replica where the manufacturer may have neglected to apply the markings, which in itself is a bad sign.
Decoding codes and symbols on disks
To read information correctly, you need to understand exactly how it is encoded. The standard marking line may look intimidating to the untrained eye, but in fact it contains a logical sequence of parameters. Example of typical marking: 7.5 Jx16 H2 ET 35 5x112 DIA 66.6.
In this line we are interested in the value after the letters ET. However, it is important not to confuse it with other numbers. For example, the number 16 indicates the diameter in inches, and 5x112 indicates the number of holes and the diameter of their location. Disc offset is always specified in millimeters and can be positive, zero or negative.
Sometimes, instead of the abbreviation ET (derived from the German Einpresstiefe), you can find the English designation Offset or just OF. The essence does not change from this. There are also disks where the parameter is indicated by a dash, for example ET-20, which indicates a negative offset characteristic of an extended track.
>R16
| Labeling parameter | Designation | Unit of measurement | Example value |
|---|---|---|---|
| Rim width | 7.5J | Inches | 7.5 |
| Disc diameter | Inches | 16 | |
| Disc offset | ET / Offset | Millimeters | 35 |
| Bolt pattern | PCD / 5x112 | Millimeters | 5x112 |
Particular attention should be paid to discs with double offset markings, which is found on universal models. In this case, you can see an inscription like ET 35/40. This means that the disc has offset mounting holes, allowing it to be mounted with two offset options depending on whether you use external or internal bolts.
Use a magnifying glass or macro mode on your smartphone camera to examine worn markings on old discs. Sometimes the light of a flashlight at an acute angle helps to reveal the relief of the numbers.
Searching for data in vehicle technical documentation
If physical inspection of the disc is not possible or the markings are unreadable, the next step is to consult the documentation. Instructions for use The (manual) of your car is the primary source of truth. The “Technical Specifications” or “Wheels and Tires” sections always indicate the parameters recommended by the manufacturer.
Information can also be found on a special plate (nameplate), which is glued to the car body. It is usually located on the body pillar in the driver's door opening, at the end of the door or under the hood. This sticker, in addition to the tire pressure, often indicates the tire and wheel sizes allowed for a given model.
However, it is worth remembering that the nameplate indicates the parameters for standard disks. If the previous owner installed non-standard wheels, the data on the plate may not match what is on the car right now. In this case, you should only trust the markings on the disk itself or the measurement results.
- 📘 Open the “Specifications” section in the car’s instructions.
- 🚪 Inspect the sticker on the driver's side B pillar.
- 📄 Check the service book if it contains notes about replacing disks.
- 💻 Find the car's VIN code and use online spare parts catalogs.
Modern electronic spare parts catalogs allow you to VIN code determine which wheels were installed on the vehicle by the manufacturer. This is especially useful if paper documentation is lost. The catalog will indicate the exact part number of the disk, knowing which you can find a drawing of it with all dimensions, including offset.
Why is it important to respect the factory offset?
The factory ET parameter is calculated by engineers taking into account the load on the suspension, the operation of the steering mechanism and aerodynamics. Deviation from the norm changes the rolling arm, which affects the stability and service life of chassis parts.
Using online directories and databases
In the digital age, there is no need to guess or make complicated measurements if you have access to the Internet. There are many specialized sites and applications that contain databases of disk parameters for thousands of car models. Just enter the make, model and year of manufacture to get a list of suitable sizes.
Popular resources allow you to filter results not only by diameter, but also by offset. You can see the range of valid values, for example, ET 30-38. This means that the manufacturer allows the installation of discs with an offset within these limits without negative consequences for the design of the car.
However, you should be careful and double-check data from unofficial sources. Databases may contain errors, especially for rare or restyled versions of models. Cross-validation information on several authoritative portals will help minimize the risk of error.
⚠️ Attention: Do not rely blindly on the advice of forums or sellers in stores. The phrase “it will get up as it is” often ignores the nuances of how the suspension works when the car is fully loaded. Always check official data.
When using online catalogs, pay attention to seasonality. Winter wheels often have different parameters than summer ones to allow for the installation of snow chains or to compensate for changes in suspension geometry. Therefore, for a winter kit it is better to look for a separate specification.
Methodology for independently calculating disc offset
In a situation where the marking is completely absent and there is no documentation, the only proven method remains - mathematical calculation. To do this, you will need to remove the disc from the car, lay it flat on a flat surface with the outer side facing up and arm yourself with a ruler or caliper.
The essence of the method is to measure two quantities: the total width of the disk and the distance from the mating plane to the edge of the rim. The calculation formula is simple: ET = A - B, where A is half the total width of the rim, and B is the distance from the mounting plane to the inner edge of the rim (on the car side).
☑️Measuring tools
First, measure the total width of the rim (not the tire!) in millimeters. We divide the resulting number in half - this is value A. Then we turn the disk over or use a flat rod thrown over the disk to measure the distance from the plane of contact with the hub to the edge of the rim - this is value B. The difference between half the width and this distance will give the desired offset.
An error of 1-2 millimeters when measuring yourself is considered acceptable, but try to be as accurate as possible.
- 📏 Measure the overall width of the rim (B_width) in mm.
- ➗ Divide the width in half (A = B_width / 2).
- 📐 Measure the distance from the mating plane to the edge of the disc (B).
- ➖ Subtract B from A to get the ET value.
If the resulting value is positive, it means that the mating plane is shifted outward from the center of the disk. If it’s negative, go inside. Zero offset means that the mounting plane coincides with the central axis of the disk. This method requires precision, but gives results when other methods are not available.
Self-measuring is a last resort. It requires wheel removal and tools, but is the only way to find out ET on unmarked rims.
Impact of departure on operation and safety
Understanding where the offset is indicated is meaningless without understanding why this parameter is so important. Changing the disc offset directly affects the position of the wheel relative to the wheel arch and suspension elements. Even a small deviation from the standard value can have serious consequences.
As the offset decreases (the disk becomes “wider”), the wheel protrudes outward. This improves visual perception and can slightly improve cornering stability by widening the track. However, this increases the load on the wheel bearing and changes the rolling arm, which can lead to instability of the steering and accelerated tire wear.
As the offset increases, the disc goes deeper into the arch. This risks causing the inner part of the disc or the tire itself to touch the suspension elements, brake caliper or shock absorber. This is especially critical when the car is fully loaded or when going over bumps when the suspension is compressed.
⚠️ Attention: Installing disks with an offset that differs from the standard one by more than 5 mm may lead to denial of warranty service for the chassis. It also poses a real safety hazard at high speeds.
Owners should avoid installing wheels with parameters that do not comply with the vehicle manufacturer's recommendations. If you really want to install non-standard wheels, use spacers, but only certified and correctly calculated ones, although this also makes changes to the design of the vehicle.
Thus, searching for ET markings is not just an academic interest, but a necessary procedure to ensure safety. Always check this setting before purchasing new drives and compare it with the information in the documentation.
Is it possible to correct the offset with spacers?
Spacers allow you to change the offset of the disc, but their use increases the load on the mounting bolts and the hub. Thin spacers (up to 10mm) are often considered relatively safe if the bolts are replaced with longer ones, but this can be legally controversial.
What to do if the ET marking is completely erased?
If the markings are not readable, the only option is to remove the wheel and take measurements using the method described in the section on self-calculation. You can also try to find the original catalog number of the disk (if it is stored elsewhere) and enter it in the manufacturer’s database.
How critical is an offset deviation of 3-4 mm?
A deviation within 3-5 mm is usually considered acceptable for most civilian vehicles and does not require replacement of the hub bolts. However, for sports cars with rigid suspension, even 2 mm can make a difference. It is best to stay within the range specified by the car manufacturer.
Does offset affect speedometer readings?
The offset (ET) itself does not affect the wheel circumference, so it does not affect the speedometer readings. Speed and readings are affected by changes in tire or wheel diameter. However, changing the offset can indirectly affect acceleration dynamics due to changes in the load on the transmission.
Where else can I find information besides the disc and PTS?
Information can be found on the website of the disc manufacturer (if the brand is known), in specialized car clubs for a specific model, as well as on forums where owners share successful experiences in installing non-standard sizes. But always double-check this data with measurements.