Wheel offset (ET) and how to choose it

Wheel ET Explained: Understanding Offset

Shopping for wheels can raise a few mysterious questions if you are new to it. Bolt pattern? Offset? Centre bore? What should you look out for so your car does not end up looking like an underfed shopping trolley? In the end, it is all fairly straightforward.

When looking for wheels that fit your car, a good starting point is to either:  a) Check the diameter, width and offset (ET) of your current wheels  or  b) Check a database to find out which wheels your car originally came with from the factory.

You can easily check this on the free website Wheel-size.com.

Once you select your car model, you will see a page like the one below, where we can examine the wheel specifications of your Bugatti Chiron. Naturally.

This tells us that the original wheels have the following specifications:

• The front wheels are 20″ in diameter, 10.0″ wide and have an offset of ET55. (Tyres: 285/30)
• The rear wheels are 21″ in diameter, 13.5″ wide and have an offset of ET71.5. (Tyres: 355/25)
• The bolt pattern used to mount the wheels to the car is 5×130. (PCD)
• The wheels are secured with wheel bolts rather than wheel nuts.
• The bolts have an M14 thread with a 1.5 mm pitch.
• The wheel centre bore is 71.60 mm.

In most cases, this information would allow you to buy wheels that fit just like the originals. However, when buying aftermarket wheels, the idea is often to tweak these dimensions a little.

Bolt Pattern and Centre Bore

The bolt pattern describes the number and arrangement of the bolt holes used to mount the wheel to your car. The wheel and the car must have matching bolt patterns. Most of our wheels are available in several bolt patterns or can be drilled to order.

The wheel centre bore simply needs to be large enough — we always supply the appropriate spigot rings to match the bore precisely to your car’s hubs.

Wheel Offset (ET)

Wheel offset, also known as ET, determines the position of the wheel’s mounting surface relative to its centreline. 

As shown in the illustration, an offset of ET0 means the wheel’s mounting surface sits exactly on its centreline.

Changing the offset works as follows:

• The higher the positive offset, the further inwards the wheel sits.
• The lower the positive offset — or, in some cases, the more negative it becomes — the further outwards the wheel sits in the wheel arch.
• Offset is measured in millimetres. If you replace an ET40 wheel with an otherwise identical ET30 wheel, it will sit 10 mm further out in the wheel arch.

NOTE: Remember that offset is measured from the wheel’s centreline, so wheel width also affects the fitment. Try comparing different dimensions using the JR Wheels offset calculator below.

How Do Wheel Spacers Affect Offset?

Wheel spacers sit between the wheel and the hub, moving the wheel outwards. If your wheel has an offset of ET40 and you add a 15 mm spacer, its effective offset becomes ET25.

Ideally, choose wheels with the correct offset from the start to avoid needing spacers unnecessarily.

How Should You Choose Your Wheel Dimensions?

The Short Version

A wide wheel sitting flush with the wheel arch looks good, but make sure the tyre has enough clearance inside the arch throughout the suspension’s full range of travel. This matters for safety too. A setup that looks great when parked may behave very differently on the road.

If you are unsure, remember that a spacer can move a wheel further out — moving it inwards would require machining.

Choose wheel and tyre sizes that work together. Tyre sizing is partly a matter of taste, but stay within the approved tyre sizes for your wheel width.

Especially on performance cars, remember that the brake calipers also need enough room inside the wheels. (Brake clearance) The spoke design can affect clearance, so check compatibility before buying if necessary, even when the width and offset match the original wheels.

When increasing wheel width, remember that the clearance between the inside of the wheel and the shock absorber or suspension arms may also decrease. Make sure there is enough space, and use the wheel calculator to compare your options.

The Longer Version

What is scrub radius? How are backspacing and wheel offset measured? How does the wheel’s rim profile affect its characteristics?
Click here to read our follow-up articles.

Wheel Offset Calculator

Click below to open the wheel calculator provided by JR Wheels and compare different dimensions.

Any Questions?

We are happy to help. Get in touch via WhatsApp, for example, and let’s talk wheels.