How to accurately find out the offset of an ET disc on alloy wheels: from markings to tape measure

Disc offset (ET) is one of the most critical parameters when selecting wheels, which directly affects the safety and controllability of the car. An error of just 5-10 mm can lead to premature suspension wear, tire friction on the arch, or even loss of control at high speed. This problem is especially relevant for alloy wheels, where manufacturers do not always apply markings in standard places, but can be visually distinguished ET35 from ET45 without measurements it is impossible.

In this article, we will analyze all the working methods for determining overhang - from decoding factory markings to independent measurements using improvised tools. You will learn where to look for the parameter ET on the rim, how to measure it correctly with a tape measure or caliper, and why you can’t trust just the appearance of the wheel. We will also provide compatibility tables for popular brands and models of cars where the offset is critical for the correct operation of the suspension.

What is ET disc flop and why you can't ignore it

Departure (ET, from him. EinpressTiefe) is the distance between mating plane disk (the one that presses against the hub) and imaginary center line rim. It is measured in millimeters and can be:

• 🔹 Positive (for example, ET40) - the mating plane is shifted to the outer side of the disk.
• 🔹 Zero (ET0) - the plane coincides with the center line.
• 🔹 Negative (for example, ET-15) — the plane is shifted inward, closer to the body.

Ignoring the crash is fraught with:

• 🚗 Accelerated wear wheel bearings and shock absorbers (at lowered ET the load on them increases by 20–30%).
• 🔧 Tire friction about the fender liner or suspension elements when cornering or under load.
• ⚠️ Unpredictable handling: The car may “steer” to one side, especially at high speeds.

⚠️ Attention: On some vehicles (for example, BMW E39 or Audi A4 B7) even the minimum departure deviation from the standard one (ET35 instead of ET38) leads to system activation ESP on turns. The manufacturer sets a tolerance of no more than ±2 mm!

In this case, visually the disk with the incorrect ET may look normal - the wheel does not stick out beyond the arch, the gaps seem sufficient. But you can’t fool physics: the displacement of the center of mass of the wheel relative to the hub changes the vectors of forces acting on the suspension.

Method 1: Looking for ET offset in the disk markings

The fastest and most accurate method is to read the parameter ET in the factory markings, which are applied to the inside of the disc. Usually it looks like this:

7.5J x 17 H2 5x112 ET45 DIA 66.6

Explanation:

• 📏 7.5J — rim width in inches.
• 📏 x17 — landing diameter.
• 🔄 H2 — the number of humps (protrusions for tubeless tires).
• ⚙️ 5x112 - number of holes and diameter of their location (PCD).
• 🎯 ET45 — disc offset (the desired parameter!).
• ⭕ DIA 66.6 - diameter of the central hole (DIA).

Where to look for markings:

1. Remove the wheel from the car (a jack and a wheelbrace will suffice).
2. Clean the inside of the disc from dirt (the markings may be hidden under a layer of dust or rust).
3. Look for an engraving or sticker on backs of the knitting needles or near the center hole. On alloy wheels it is often written in small print.

⚠️ Attention: On Chinese discs of unnamed brands (for example, Replay or Alutec) markings may be applied with paint, which will wear off over time. In this case, use measurement methods (see Methods 3–5).

Method 2: Determine offset by VIN code or vehicle documents

If the disk is standard or you want to know recommended departure for your model, use official sources:

To accurately search by VIN, use the following services:

• 🔗 VIN decoder Behind the Wheel (free, for Russian cars).
• 🔗 VinFreeCheck (international base).

⚠️ Attention: On some vehicles (for example, Mercedes-Benz W204) disc offset depends on the configuration. Yes, for AMG packages often used ET30–ET35, and for standard versions - ET45–ET50. Check details for specific option code (for example, 481 for sports suspension).

Method 3: We measure the disc offset with a tape measure (method for beginners)

If there are no markings and the documentation is lost, you can measure ET on your own. You will need:

• 📏 Tape measure or caliper (more precisely).
• 🪛 A straight metal ruler or square.
• 🧲 Magnet (to search for the mating plane).

Step by step instructions:

1. Remove the wheel and clean the mating surface of the disc from dirt.
2. Place the disc on a flat surface face down (knitting needles up).
3. Find the center of the disk:
   • Measure rim width (A) — the distance between the inner edges of the sides (for example, 203 mm for an 8J disk).
   • Divide the width in half: A/2 = 101.5 mm.

ET = (A/2) – B

Typical measurement errors:

• ❌ Side not taken into account for tubeless tires (Hump) - measure strictly to grounds rim.
• ❌ Crooked disk - if it is deformed, the result will be inaccurate.
• ❌ Using a Flexible Ruler - take a metal one to avoid sagging.

Method 4: Use a caliper for accurate measurements

For discs with complex geometries (for example, BBS CH-R or OZ Racing Ultraleggera) a tape measure is not enough - knitting needles can interfere with measurements. Help here caliper with depth gauge:

Algorithm:

1. Install the disk face up on a flat surface.
2. Place the caliper on mating plane and measure the distance to outer edge of rim (C).
3. Turn the disc over and measure the distance from the mating plane to inner edge of rim (D).
4. Calculate the offset using the formula:

   ET = (C – D)/2

   For example, if C = 120 mm, and D = 80 mm, then ET = (120 – 80)/2 = 20 mm (ET20).

Critical detail: on rims with asymmetrical spoke shapes (e.g. Enkei PF05) the mating plane must be found by landing bolts, and not along the visual edge!

Method 5: Refer to the disk manufacturer's database

If the disc is branded (for example, BBS, OZ, Ronal), departure can be found by article number or models on the official website. Examples:

• 🔗 BBS: Enter the disk model (for example, CH-R 19x8.5) → download the technical data sheet (ET indicated in the table).
• 🔗 OZ Racing: “Wheel Configurator” section → select a car model → look at the acceptable values ET.
• 🔗 Ronal: in the catalog filter by parameter Offset (mm).

For Chinese discs (for example, Replica or Lenso) look for data on the following sites:

• 🛒 AliExpress (in the product description).
• 📦 eBay (Section “Specifications”).

⚠️ Attention: On some replicas (for example, copies BBS LM) real ET may differ from declared by 3–5 mm. Always double check with measurements!

ET Offset Compatibility Chart for Popular Cars

Below is a list of models where overhang is critical to safety. The data is relevant for standard disk sizes (not taking into account lifts or suspension lowering).

For an accurate selection, use the catalogs:

• 🔧 Wheel-Size (database by brands and models).
• 📊 Alloy Wheels Direct (filter by ET and PCD).

Common mistakes and how to avoid them

It’s easy to make a mistake with a flight if you don’t know the nuances. Let's look at typical pitfalls:

• 🔴 Trust the seller's word:

  On the used disk market, they often slip in “universal” options with ET0 or ET20, which “goes with everything.” In fact, such a disk may be dangerous for your model.

• 🔴 Ignore hubs (spacers):

  If the disc offset is less than standard (for example, ET30 instead of ET45), some install spacers. This is a temporary solution - it increases the load on the wheel bearing.

• 🔴 Confusing ET with backspacing:

  Backspacing (or BS) is the distance from the mating plane to external edges of the disk. It is sometimes mistaken for relegation. Conversion formula:

  ET = Backspacing – (Rim Width / 2)

How to check that the offset is selected correctly:

1. Install the wheel and scroll it — it should not cling to the arch or suspension elements.
2. Drive along speed bump — are there any extraneous sounds (creaks, knocks).
3. At speed 80–100 km/h let go of the steering wheel - the car should not pull to the side.

FAQ: Answers to frequently asked questions

New ET = Standard ET + (Lower Amount / 2)