Wheels & Tires

What is ET / Offset?

One of the most important factors to consider when changing to a different set of wheels—besides wheel width, diameter, and tire size—is the wheel's ET value. ET refers to the wheel's offset, also commonly known simply as offset.

Simply put, the ET value determines where the wheel sits in relation to its mounting surface against the wheel hub. This directly affects how far inward or outward the wheel is positioned when mounted on your vehicle.

A positive ET (offset) generally means that the wheel sits further inward toward the suspension, while a negative ETmoves the wheel further outward from the hub. An ET of 0 means that the mounting surface is located exactly on the wheel's centerline.

The correct ET for your vehicle varies from model to model, as it depends on the position of the wheel hub relative to the bodywork, as well as the width of the wheel. Proper wheel fitment is also affected by the width and aspect ratio of the tires you choose.

A good rule of thumb is to check the ET of your vehicle's original wheels and calculate from the manufacturer's specifications. If possible, it's also beneficial to test-fit different wheel combinations on the vehicle for comparison. Keep in mind that ET is directly related to the width of the wheel.

A useful website that we use for both visual and mathematical wheel fitment comparisons is:

http://www.willtheyfit.com

How do you measure the width of a wheel?

To calculate a wheel's offset (ET), you first need to know the wheel's width.

One common source of confusion is that the wheel's overall physical width is not the same as the width used when calculating ET. Just like wheel diameter, the official wheel width is measured from bead seat to bead seat, not from outer edge to outer edge. This means that the width stamped on the wheel refers to the distance between the tire bead seats rather than the wheel's total outside width.

How do I calculate a wheel's ET (Offset)?

To calculate a wheel's ET (offset), you need two measurements:

• The wheel's overall width (to determine the wheel's centerline).
• The distance from the back edge of the wheel to the hub mounting surface.

Start by placing the wheel face down on a flat, hard surface. Measure the distance from the floor to the highest point of the wheel. This measurement is (C), the wheel's overall width, with or without a tire mounted. A straight steel ruler or another straight edge is usually helpful for taking this measurement.

Next, measure from the same highest point down to the wheel's hub mounting surface (B). Then subtract half of the wheel's overall width (C/2) from measurement B.

The goal is to determine the wheel's true centerline. This remains the same whether you measure with or without a tire, provided the tire extends equally on both sides of the wheel. By subtracting half of the wheel's width from the distance between the back edge of the wheel and the mounting surface, you obtain the wheel's ET (offset) value (A). This indicates how far the mounting surface is positioned from the wheel's centerline, either positively or negatively, measured in millimeters.

How do I read and choose the correct tire?

To keep things simple, we'll focus on tire dimensions and how to read them.

In the example below, the tire size is 225/50 R17 98H. The most relevant numbers in this case are 225, 50, and 17.

• 225 is the tire's width in millimeters, measured from sidewall to sidewall.
• 50 is the tire's aspect ratio (profile). It indicates that the tire's sidewall height is 50% of its width (in this example, 50% of 225 mm = 112.5 mm).
• 17 is the tire's (or more accurately, the wheel's) diameter in inches. This tire is designed to fit a 17-inch wheel.

The tire in this example is therefore 225 mm wide, has a 112.5 mm sidewall height, and fits a 17-inch wheel.

In practical terms, this means that a tire's aspect ratio depends entirely on its width. You can maintain approximately the same overall wheel-and-tire diameter by increasing the wheel diameter (inches) while decreasing the tire profile. When fitting new wheels, remember to take both the tire width and aspect ratio into account to ensure proper fitment.

Bolt Pattern and Center Bore

Even more important than the wheel's ET (offset) and tire size is the bolt pattern and, to some extent, the center bore. The correct bolt pattern is essential for the wheels to fit your vehicle.

Most wheels have either a 4-lug or 5-lug bolt pattern, and there are numerous bolt pattern combinations specific to different vehicle makes and models. The method of measuring the bolt pattern differs slightly depending on whether the wheel has 4 or 5 lug holes.

On a 4-lug wheel, the bolt pattern is measured from the center of one bolt hole to the center of the bolt hole directly opposite it. Alternatively, you can measure from the inside edge of one bolt hole to the outside edge of the opposite bolt hole, which gives the same result.

A 5-lug wheel is measured differently because the bolt holes are not directly opposite each other. The easiest method is to measure from the center of one bolt hole to the outer edge of the third bolt hole (counting from the hole you're measuring from). While this method is commonly used, it is not always perfectly accurate. For the most precise measurement, use the method described below.

With this more accurate method, measure:

• The wheel's center bore diameter (A).
• The diameter of a bolt hole (B).
• The distance from the inside edge of a bolt hole to the edge of the center bore (C).

Then calculate:

A + B + C + C

If the wheel has a 6-lug or 8-lug bolt pattern, it is measured the same way as a 4-lug wheel.

The center bore is also specific to your vehicle model and wheel hub. The most important requirement is that the wheel's center bore is at least as large as the hub diameter on your vehicle. If the center bore is larger, hub-centric rings (centering rings) can be used to fill the gap, eliminate vibrations, and help center the wheel correctly. However, the majority of the wheel's centering is still achieved by the wheel bolts or lug nuts, which are available in a variety of thread types, seat designs (conical, ball, etc.), and dimensions to ensure proper wheel alignment.

Thomas Kula

Retrospeed
info@retrospeed.fi