Disc offset (ET) is one of the key parameters that affects the safety and controllability of the car. An incorrectly selected offset can lead to accelerated wear of the suspension, wheel bearings and even loss of control over the car at high speed. Many car owners mistakenly believe that it is enough to select wheels by diameter and bolt pattern, but even a minimal deviation of ET from factory recommendations of ±5 mm can critically change the wheelbase geometry.

In this article, you will learn how to independently measure and calculate the disc offset using available tools, what formulas to use, and what mistakes are most often made when taking measurements. We'll also look at why you can't trust disc labels without checking them and how to correctly interpret the results for different types of cars - from cars to SUVs.

If you're planning on installing custom wheels or just want to check that your current wheels match the factory specs, these instructions will help you avoid costly mistakes. And for those who prefer visual diagrams, we have prepared a table of tolerances and examples of calculations for popular car brands.

📊Have you ever measured the disc offset yourself?
Yes, regularly
Tried it, but not sure about accuracy
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I didn't know you could do this yourself

What is ET disc ejection and why does it need to be checked?

Disc offset (indicated as ET from German EinpressTiefe) is the distance between the mating plane of the disk (the one that presses against the hub) and the imaginary central axis of the disk. It is measured in millimeters and can be:

  • 🔹 Positive (ET30, ET45) - the mating plane protrudes outward relative to the center;
  • 🔹 Zero (ET0) - the plane coincides with the central axis;
  • 🔹 Negative (ET-10, ET-25) - the plane is recessed inward.

The vehicle manufacturer always indicates the permissible ET range in the technical documentation. For example, for Volkswagen Golf VII standard front wheel offset - ET45-50, and for Toyota Land Cruiser 200 — ET25. Deviation from these values leads to:

  • 🔧 Increased load on wheel bearings;
  • 🔧 Changing the rolling arm, which impairs controllability;
  • 🔧 Wheel-arch contact when turning or under load.

Reach is especially critical for vehicles with independent multi-link suspension (for example, Audi A4 or BMW 3 Series). Here, even a minimal ET offset of 3-5mm can cause uneven tire wear and vibration in the steering wheel. At the same time, for cars with suspension MacPherson (for example, Renault Logan) tolerances are usually wider.

⚠️ Attention: On some discs, offset is indicated with an error of ±2 mm. If you see the marking ET42±2, this means that the actual value may vary from ET40 up to ET44. Always check the exact parameters with the seller or measure yourself.

Tools for measuring disc offset at home

To measure the reach yourself, you do not need professional tools. The following set is sufficient:

  • 📏 Ruler or caliper (preferably with a depth gauge);
  • 📐 Square (for perpendicular measurements);
  • 🔨 Flat strip or metal plate (length not less than 30 cm);
  • 📝 Paper and pencil for records;
  • 🧲 Magnet (optional, for fixing the rail).

For accuracy it is recommended to use digital caliper with a division value of 0.01 mm (for example, models Mitutoyo or Shinwa). If it is not there, a regular metal ruler will do, but the measurement error will increase to ±1–2 mm.

Also useful level (bubble or laser) to check the perpendicularity of the rack relative to the disk. If the drive is already installed in the car, you can use flat object with a magnet (such as a metal ruler) to secure it to the rim.

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Before measuring, clean the mounting surface of the disk from dirt and rust - even small particles can distort the result by 1-3 mm.

Step-by-step instructions: how to measure disc offset ET

There are three main methods for calculating the offset: using a rod, a caliper and mathematically (using a formula). We will analyze each of them with examples.

Method 1: Measuring with a staff (with the disc removed)

This method is suitable if the disk is not yet installed on the car.

  1. Place the disc on a flat surface mating plane up.
  2. Place the rack across the rim so that it touches the outer edge of the rim.
  3. Measure the distance from the mating plane to the bottom edge of the rail (parameter A).
  4. Turn the disk over and repeat the measurement (parameter B).
  5. Calculate the offset using the formula: ET = (B - A)/2 - C, where C — disk width (indicated in the marking, for example, 7.5J = 190.5 mm).

Method 2: Using a Vernier Caliper (Precision Method)

This method gives a minimum error (±0.5 mm) and is suitable for disks of any design.

  1. Place the disk vertically (can be secured in a vice).
  2. Measure the distance from the mating plane to inner edge of the rim (parameter X).
  3. Measure full rim width (parameter Y).
  4. Calculate departure: ET = X - Y/2.

Method 3: Mathematical calculation (using formula)

If you know rim width (W) and distance from the mating plane to the inner edge (Backspacing, BS), use the formula: 

ET = BS - W/2

For example, for a disk with W = 8 inches (203.2 mm) and BS = 150 mm: 

ET = 150 - 203.2/2 = 150 - 101.6 = 48.4 mm

Make sure that the disc is not deformed | Clean the mating plane from dirt | Use a precise tool (error ≤1 mm) | Repeat measurements 2-3 times to check

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The table below shows standard offset values for common models. Please note that for some vehicles (eg Mercedes-Benz or Porsche) tolerances are stricter than for mass brands.

Make and model Standard ET (mm) Tolerance (mm) Notes
Volkswagen Passat B8 38–45 ±3 For versions with all-wheel drive (4Motion) ET strictly 45
Toyota Camry XV70 35–40 ±2 For the hybrid version, the approval is already limited to ET38
Hyundai Tucson (TL) 30–38 ±4 For wheels R18–R19, ET35 is recommended
BMW 5 Series (G30) 15–25 ±1 Negative offset (ET15) only for M package
Lada Vesta 35–40 ±5 Allowed wheels with ET30 for tuning versions

If your model is not in the table, find the data in instruction manual (section "Wheels and Tires") or on the official website of the manufacturer. For vehicles with air suspension (for example, Mercedes S-Class) the offset may depend on the position of the body - in this case, dynamic balancing is required after installing new disks.

Common mistakes when calculating departures and how to avoid them

Even experienced car owners sometimes make mistakes that lead to incorrect selection of wheels. Here are the most common of them:

  • 🔴 Ignoring disk markings. Inscription ET38 does not always correspond to the real offset - especially for non-original disks. Always double check your measurements.
  • 🔴 Disk width not taken into account. Formula ET = BS - W/2 only works if W measured accurately. An error of 0.5 inches will result in an error of 6-7 mm.
  • 🔴 Measurements on the installed wheel. If the disc is already on the car, it is almost impossible to measure the offset without removing it - the brake mechanisms get in the way.
  • 🔴 Using a Wooden Ruler. The wood may warp, which will skew the results. Use only metal tools.

Another typical problem is confusion between Backspacing (BS) and Elevation (ET). These options are related but not identical:

  • 🔹 Backspacing (BS) — distance from the mating plane to inner edge disk.
  • 🔹 Departure (ET) — distance from the mating plane to central axis disk.

For example, for a disk with BS = 160 mm and width 8 inches (203.2 mm): 

ET = 160 - 203.2/2 = 160 - 101.6 = 58.4 mm

But if the seller indicated ET45, this means the disc is not suitable for your car.

What to do if the offset does not match the factory one?

If the difference between the factory ET and the actual disc offset is more than 5 mm, there are three options:

1. Use spacers (for positive ET only). For example, if you need ET45, but the disk has ET35, you can install a spacer with a thickness of 10 mm.

2. Choose another drive. This is the most reliable, but also the most expensive method.

3. Consult a specialist. In some cases (for example, for SUVs) deviations of up to 10 mm are allowed, but an alignment check is required at the wheel alignment stand.

⚠️ Attention: Spacers for adjusting offset change the load on the wheel bearing. Their thickness should not exceed 15 mm for passenger cars and 25 mm for SUVs. Otherwise, the bearings must be replaced with reinforced ones.

How disc offset affects handling and safety

Changing the offset even by 5–10 mm can radically change the behavior of the car on the road. Let's look at the consequences using examples:

  • 🔄 Decrease in ET (disc “bulges” out):
    • ✅ The track increases, which improves stability on straight lines.
    • ❌ The load on bearings and steering rods is increasing.
    • ❌ The wheel can hit the arch when fully loaded or on bumps.
  • 🔄 Magnification ET (the disk is “recessed” inside):
    • ✅ The load on the suspension is reduced.
    • ❌ Handling deteriorates at high speeds (the car “floats”).
    • ❌ Contact of the wheel with elements of the brake system is possible.

For sports cars (eg Subaru WRX or Nissan GT-R) are often used negative offset (ET-10...ET0) to widen the track and improve grip in corners. However, this requires suspension modifications and regular wheel alignment checks.

On SUVs (eg. Jeep Wrangler or Toyota LC79) negative offset allows you to install wider tires without a body lift. But in this case, it is necessary to strengthen the axles and steering.

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For 90% of passenger cars, the optimal offset is from ET30 to ET50. Deviations are permissible only during tuning with a complete redesign of the suspension.

Checking the offset on an installed wheel: myths and reality

Many car owners try to measure the offset without removing the wheel from the car. This is possible, but with serious caveats:

  • ✅ The method works only if known exact rim width and tire size.
  • ❌ Error can reach 10–15 mm due to tire deformation under the weight of the car.
  • ❌ Cannot be taken into account disk deflection when tightening bolts.

If you still need to roughly estimate the offset without removing the wheel, follow this algorithm:

  1. Jack up the car and remove the wheel (but do not disconnect it from the hub).
  2. Measure the distance from the mating plane to inner edge of the rim (BS).
  3. Find out the width of the disk (W) from the marking (for example, 7.5J = 190.5 mm).
  4. Calculate ET using the formula ET = BS - W/2.

For more accurate results, use laser levelto check the perpendicularity of the measurements. However, even in this case the error will be at least 3–5 mm.

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If you buy used wheels, be sure to check the offset on all four wheels. It is not uncommon for discs from the same set to have different ET due to manufacturing defects or repairs.

FAQ: Frequently asked questions about ET disc ejection

Is it possible to install a disc with an offset of ET30 instead of ET45?

Theoretically it is possible, but this will lead to an outward displacement of the wheel by 15 mm. Consequences:

  • The load on wheel bearings and steering tips will increase.
  • It is possible for the wheel to hit the arch when the steering wheel is turned completely.
  • Wheel alignment adjustment will be required.

This is acceptable for temporary driving (for example, on a road trip), but it is not recommended to constantly operate a car with such a deviation.

How does offset affect tire wear?

Incorrect offset changes the wheel alignment angles, which leads to:

  • Uneven wear tread (for example, “sawtooth” wear at ET is less than factory).
  • Accelerated abrasion inner or outer edge of the tire.
  • Vibrations at speeds over 100 km/h.

Example: on Skoda Octavia with ET40 instead of ET48 the tires wear out 30% faster.

How does the ET offset differ from the DIA parameter?

ET (departure) and DIA (center hole diameter) - these are different parameters:

  • ET — distance from the mating plane to the center of the disk.
  • DIA - diameter of the hole for the hub (for example, 57.1 mm for BMW).

Both parameters are critical: an incorrect DIA will lead to wheel runout, and an incorrect ET will lead to changes in suspension geometry.

Is it possible to make spacers yourself to adjust the offset?

Technically yes, but it's dangerous:

  • Spacers must be made from aviation aluminum or steel.
  • The thickness of the spacer should not exceed 15 mm for passenger cars.
  • Mandatory dynamic balancing after installation.

Homemade spacers made from scrap materials (for example, steel plates) can lead to wheel coming off while driving.

What is the offset of wheels marked ET0?

ET0 means that the mating plane of the disk coincides with its central axis. Such disks are often used for:

  • Tuned cars (for example, Honda Civic Type R).
  • SUVs with wide wheels (for example, Ford Raptor).
  • Racing cars where the maximum track is important.

For most production cars, ET0 is not suitable - it increases the load on the suspension by 20-30%.