Have you ever looked at the back of an alloy wheel and wondered what all those numbers and letters actually mean?

You might see something like 20x8.5J ET40 5x120 CB66.9 stamped, cast or engraved into the wheel. To an experienced wheel technician, this specification tells an important story about the wheel. But for many car owners, it can look like a random collection of numbers.

Understanding these markings is one of the easiest ways to become more confident when choosing aftermarket wheels. More importantly, it can help you avoid expensive fitment mistakes.

In this guide, we will break down the most common wheel markings and explain what each specification means, using Holden Commodore and HSV applications as practical examples.

1. Where to Find Your Wheel Specifications

Wheel specifications can appear in several different places depending on the manufacturer and wheel design.

• Wheel spokes or inner barrel – Some wheels have their specifications cast or engraved into the back of the spokes or inside the barrel.
• Back of the wheel – Diameter, width, offset and other technical information may be marked on the rear face.
• Casting or engraving – Many alloy wheels have permanent technical markings formed during manufacturing or added afterwards.
• Product specifications – New aftermarket wheels will normally have their specifications listed by the manufacturer or retailer.
• Manufacturer markings – Brand, model, certification and load-related information may also appear on the wheel.

Not every manufacturer uses exactly the same marking format, so you may not always see every specification in one place.

However, if you come across a marking such as:

you can break it down into several key specifications.

Marking	Meaning
20	Wheel diameter
8.5J	Wheel width and rim profile designation
ET40	+40mm wheel offset
5x120	5-lug PCD with a 120mm pitch circle diameter
CB66.9	66.9mm centre bore

2. What Does 20x8.5J Mean?

The first part of a wheel specification usually describes the physical size of the wheel.

20 = Wheel Diameter

The 20 means the wheel has a nominal diameter of 20 inches.

This is the measurement that determines whether a tyre designed for a 20-inch wheel can be fitted. Moving from an 18-inch to a 19-inch or 20-inch wheel can also change the overall appearance, tyre sidewall height and available brake clearance.

8.5 = Wheel Width

The 8.5 means the wheel is approximately 8.5 inches wide, measured across the wheel's specified rim width.

Wheel width is particularly important because it influences:

• Which tyre widths are suitable
• Inner suspension clearance
• Outer guard clearance
• Overall vehicle stance
• Tyre sidewall shape
• How aggressive or factory-like the fitment looks

This is why 20x8.5 and 20x9.5 are not simply two wheels that are the same size with a minor difference.

The extra width changes where the inner and outer edges of the wheel sit. When combined with offset, tyre width and suspension height, that difference can have a significant effect on clearance.

What Does the "J" Mean?

The J in a specification such as 8.5J is a rim profile or flange designation. It is not another measurement of wheel width.

For most consumers, the important part is understanding that 8.5J identifies an 8.5-inch-wide wheel with a particular standardised rim flange profile.

When comparing aftermarket wheels, focus primarily on the actual diameter and width, while recognising that the letter is part of the wheel's technical specification.

3. What Does ET40 Mean?

ET40 means the wheel has a +40mm offset.

The term ET comes from the German word Einpresstiefe, which refers to the wheel's insertion depth or offset.

Offset determines the position of the wheel relative to the vehicle's hub mounting surface. Changing the offset can move the wheel further inward or outward, affecting suspension, brake and guard clearance.

For example, two wheels may both be 20x8.5, but an ET35 wheel and an ET45 wheel will not occupy exactly the same position on the vehicle.

Because offset is one of the most misunderstood parts of wheel fitment, we have covered it in detail in our previous guide:

Read more: Wheel Offset Explained: Why the Wrong Offset Can Ruin Your Holden Fitment

The important point here is that ET should always be considered together with wheel width. Looking at the offset number alone does not tell you exactly where the outside edge of the wheel will sit.

4. What Does 5x120 Mean?

Another important marking is the wheel's PCD, or Pitch Circle Diameter.

A specification such as 5x120 means:

• 5 = The wheel has five mounting holes.
• 120 = The mounting holes are positioned on a 120mm pitch circle diameter.

PCD is one of the fundamental requirements for wheel compatibility.

For example, a wheel designed for a 5x120 application is not automatically compatible with a vehicle using a different PCD simply because the wheel physically appears close to fitting.

PCD and Holden Applications

PCD is particularly important when shopping for reproduction wheels for Holden and HSV vehicles because different Holden platforms and applications can use different wheel specifications.

Before purchasing a wheel, always confirm the exact vehicle application rather than assuming that a wheel designed for one Holden model will fit another.

This is especially important when considering reproduction wheels, where a wheel may visually replicate an OEM design but be manufactured in different widths, offsets, PCDs or centre bores for different applications.

5. What Does CB or Centre Bore Mean?

CB stands for Centre Bore.

The centre bore is the diameter of the machined hole in the centre of the wheel that locates around the vehicle's hub.

For example:

The centre bore is important because it affects how the wheel locates on the vehicle hub.

What Happens If the Centre Bore Is Too Small?

If the wheel's centre bore is smaller than the vehicle's hub diameter, the wheel cannot physically slide over the hub.

This is a basic dimensional incompatibility and cannot be solved simply by tightening the wheel nuts.

What If the Centre Bore Is Larger?

An aftermarket wheel may have a larger centre bore than the vehicle hub. In applications where appropriate, a correctly sized hub-centric ring can be used to reduce the effective centre bore and help centre the wheel on the vehicle hub.

This is commonly referred to as a hub-centric fitment.

For a deeper explanation of hub-centric and lug-centric wheel mounting, see our previous guide:

Read more: Hub-Centric vs Lug-Centric Fitment

6. What About Wheel Load Rating?

Some wheels will also carry information about their load rating.

Load Rating = the maximum load the wheel is designed to support under its specified conditions.

This specification becomes particularly relevant when selecting wheels for vehicles that carry heavier loads or are used for towing, touring or other demanding applications.

For example, wheel load capacity deserves careful consideration on:

• Heavy Holden applications
• Utes
• SUVs
• 4x4 vehicles
• Vehicles regularly used for towing or carrying significant loads

It is important not to confuse wheel load rating with the tyre load index.

The wheel's load rating relates to the structural capacity of the wheel, while the tyre's load index relates to the maximum load that the tyre is designed to carry under its specified conditions. They are separate specifications and both need to be appropriate for the vehicle.

For a more detailed look at why wheel strength matters for heavier Holden applications, see:

Read more: Load Rating Secrets: Why Heavy Holdens Require Specialized Wheel Strength

7. How to Read a Complete Wheel Specification

Now let's put everything together.

Imagine you find the following specification on an alloy wheel:

Here's what each part tells you:

Marking	Meaning	Why It Matters
20	Wheel diameter	Determines the nominal wheel size and compatible tyre diameter
8.5J	8.5-inch wheel width + rim profile designation	Affects tyre selection, stance and clearance
ET40	+40mm offset	Controls the wheel's position relative to the hub
5x120	5-lug, 120mm PCD	Must correctly match the vehicle's mounting pattern
CB66.9	66.9mm centre bore	Determines how the wheel locates around the vehicle hub

However, there is one important lesson to take away from this example:

Don't judge wheel fitment from one number alone.

A wheel can have the correct diameter but the wrong PCD. It can have the correct PCD but the wrong centre bore. It can have the correct PCD and centre bore but still have an unsuitable width or offset.

Complete wheel fitment requires all the relevant specifications to work together.

8. The Six Specifications You Should Check Before Buying Wheels

When comparing aftermarket wheels, especially online, we recommend checking at least these specifications:

1. Diameter – Is the wheel the correct size for your vehicle and intended tyre?
2. Width – Is the wheel width appropriate for your tyre and available clearance?
3. Offset – Will the wheel sit in an appropriate position relative to the suspension, brakes and guards?
4. PCD – Does the mounting pattern correctly match the vehicle?
5. Centre Bore – Does the wheel properly accommodate the vehicle hub?
6. Brake Clearance – Does the wheel's internal spoke and barrel design clear the brake package?

And there is one more specification that should never be treated as an afterthought:

Tyre size.

The wheel and tyre should be considered as one complete package. A wheel that works with one tyre width or profile may produce a very different result with another.

9. Why This Matters for Holden Commodore & HSV Wheels

Holden Commodore and HSV vehicles are a great example of why wheel specifications need to be considered as a complete system.

Many Holden owners choose larger aftermarket wheels to improve the appearance of their vehicle while maintaining a practical street setup. But the correct result depends on much more than simply choosing a 19-inch or 20-inch design.

Depending on the vehicle, you may need to consider:

• VE or VF platform
• SS, SSV, Redline or HSV application
• Wheel diameter
• Front and rear wheel width
• Offset
• PCD
• Centre bore
• Factory or upgraded brake package
• Tyre width and profile
• Suspension height
• Guard and inner clearance

This is especially important when fitting reproduction wheels to HSV models equipped with larger brake packages. Two wheels can have identical diameter, width, offset and PCD on paper but still have different internal spoke profiles, which can affect brake clearance.

That's why the wheel specification is only part of the fitment equation.

10. Wheel Markings vs Actual Vehicle Fitment

One of the most common mistakes when shopping for wheels online is assuming that matching a few numbers automatically guarantees fitment.

For example:

"My car needs 20-inch wheels, so any 20-inch wheel should fit."

Not necessarily.

Wheel diameter is only one part of the specification.

Another common mistake is:

"The PCD is correct, so the wheel will fit."

Again, not necessarily.

Correct PCD does not automatically guarantee the correct offset, centre bore, brake clearance or guard clearance.

This is why professional wheel fitment should always consider the complete specification rather than matching one number from the original wheel.

11. A Simple Wheel Specification Checklist

Before ordering a new set of alloy wheels, take a moment to check the following:

Check	Question
Diameter	Is the wheel diameter suitable for the vehicle and tyre?
Width	Is the wheel width appropriate for the tyre and clearance?
Offset	Will the wheel sit correctly inside the guards and away from suspension components?
PCD	Does the mounting pattern match the vehicle?
Centre Bore	Does the centre bore correctly fit the vehicle hub?
Brake Clearance	Will the spokes and barrel clear the brake package?
Tyres	Is the tyre width and profile suitable for the selected wheel?
Load Rating	Is the wheel's load capacity appropriate for the vehicle?

12. Understanding Your Wheel Specifications Makes Buying Easier

Wheel markings might look complicated at first, but once you understand the basic terminology, they become much easier to read.

20x8.5J tells you the wheel's diameter and width.

ET40 tells you the offset.

5x120 tells you the PCD.

CB66.9 tells you the centre bore.

And additional markings can provide information about load capacity, manufacturing standards and other technical characteristics.

But remember that understanding the numbers is only the first step.

The real question is whether those numbers work together with your specific vehicle, tyres, brakes and suspension.

For Holden Commodore and HSV owners, choosing the right combination can make the difference between a clean, factory-inspired fitment and a setup that creates unnecessary clearance or installation problems.

13. Find the Right Holden & HSV Wheels

At WheelsZone Australia, we offer a range of reproduction wheels designed around popular Holden and HSV applications.

From classic Holden-inspired designs to modern 20-inch HSV reproduction wheels, the right combination of diameter, width, offset, PCD and centre bore is essential for achieving the look and fitment you want.

Explore our collection of Holden and HSV reproduction wheels:

Frequently Asked Questions

What do the numbers on alloy wheels mean?

Wheel markings identify important specifications such as diameter, width, offset, PCD and centre bore. For example, 20x8.5J ET40 5x120 CB66.9 describes a 20-inch diameter, 8.5-inch-wide wheel with +40mm offset, a 5x120 PCD and a 66.9mm centre bore.

What does 20x8.5 mean on a wheel?

20 refers to the wheel's nominal diameter in inches, while 8.5 refers to its width in inches. The width affects tyre selection, clearance and overall vehicle stance.

What does ET40 mean on a wheel?

ET40 means the wheel has a +40mm offset. Offset determines the wheel's position relative to the vehicle's hub and should always be considered together with wheel width and the vehicle's available clearance.

What does 5x120 mean?

5x120 is the wheel's PCD. The 5 indicates five mounting holes and 120 indicates a 120mm pitch circle diameter. The PCD must correctly match the vehicle's mounting pattern.

What does CB66.9 mean?

CB66.9 means the wheel has a 66.9mm centre bore. The centre bore is the opening in the centre of the wheel that locates around the vehicle's hub.

Can a wheel with a larger centre bore fit my car?

A larger centre bore may be compatible when the wheel's other specifications are correct. In appropriate applications, a correctly sized hub-centric ring can be used to reduce the centre bore to match the vehicle hub.

Is PCD the same as centre bore?

No. PCD describes the arrangement of the wheel's mounting holes, while centre bore describes the diameter of the central locating hole. Both are separate wheel specifications.

Does the correct PCD mean a wheel will fit?

No. Correct PCD is essential, but complete fitment also depends on diameter, width, offset, centre bore, brake clearance, tyre size and suspension or guard clearance.

What is wheel load rating?

Wheel load rating refers to the maximum load the wheel is designed to support under its specified conditions. It should not be confused with the tyre's load index, which is a separate specification.

What are the most important wheel specifications to check?

The key specifications are diameter, width, offset, PCD and centre bore. You should also confirm brake clearance, tyre compatibility and wheel load rating where relevant.