The Ultimate Beginner's Guide to Alloy Wheel Terminology
Upgrading your car’s alloy wheels can completely change its look, feel and performance. If you are new to the world of alloy wheels, some of the terminology can feel overwhelming and confusing. What’s PCD? Trying to understand a “centre bore” without context can feel like quantum physics – but it really isn’t.
At G Force Styling, it is our job to know all of these things inside out, which is why we’ve created this guide to help you understand the essential terms you need to know before making your alloy wheel purchase. We’ll break down some essential terms to help you understand what they mean and even debunk some common myths along the way.
Why it is important to understand alloy wheel terminology
Ensuring you make informed decisions when choosing your alloy wheels is essential, as you must select the right choice for your vehicle, needs, and aesthetics. The good news is that once you understand the basic concepts, you won’t need to know much more – you absolutely do not need to have an engineering degree – we promise.
At G Force Styling, our website will help you navigate everything you need to know and showcase all the relevant information you need before purchasing.
Essential Alloy Wheel Terms: Decoded
PCD (Pitch Circle Diameter)
One of the most important things to know when shopping for alloy wheels is the PCD. This refers to the diameter of an imaginary circle drawn through the centre of the bolt holes on your wheels.
It is usually expressed as two numbers, such as 4x100. But what does this mean?
The first number: This is the number of bolt holes on the wheels (in the example above, this would be a 4)
The second number: This number is the diameter of the imaginary circle in millimetres (in the example above, this would be 100)
Myth buster: It is a common misconception that all 4-stud wheels will fit any 4-stud vehicle, but this isn’t the case. The stud spacing (the PCD measurements) must match your vehicle’s specifications exactly.
Wheel Offset (ET)
Offset determines how far under the arch or how far towards the outer arch a wheel will sit. It's measured in millimetres and is typically shown as "ET" followed by a number (e.g., ET49).
In order to visualise offset, imagine cutting the wheel in half; the offset is the distance between:
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The mounting face of the wheel (where it attaches to the hub)
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The centre line of the wheel’s width
Every vehicle has an optimal offset range, and ensuring this is the right size is important.
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Too low: The wheel may stick out from the side of the car
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Too high: The wheel may intrude too far under the vehicle, potentially catching on suspension components or bodywork
Can you adjust the offset with wheel spacers?
If a wheel's offset is too high for your vehicle, wheel spacers can often be used to reduce it, pushing the wheel outward to achieve proper fitment. This is completely safe and road-legal when done correctly. Make sure you get a professional to complete this process for you, otherwise you could be at risk of making your car road illegal.
If a wheel's offset is too low, there's no way to fix this issue, as the wheels will already sit too far out.
Wheel Sizing (Width & Diameter)
Wheel size is typically displayed as: Wheel diameter × Wheel width (e.g., 15 × 6.5J)
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Diameter: The overall diameter of the wheel in inches
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Width: The distance between the tyre mounting flanges on the wheel, also in inches
These measurements are crucial for determining which tyres will fit your wheels properly.
Do bigger wheels mean better performance?
A common misconception is that bigger wheels automatically improve performance. While larger wheels can enhance aesthetics and provide better grip (when paired with the right tyres), they can also negatively impact ride comfort and fuel economy. It’s important to balance style with functionality, and find what works best for you and your vehicle.
Centre Bore and Spigot Rings
The centre bore is the diameter of the centre hole in the back of the wheel. For optimal performance and safety, this hole needs to locate properly on the hub lip of your vehicle.
Most alloy wheels have a centre bore larger than most vehicle hub lips, allowing them to fit various cars. This is where spigot rings come in – these rings fit into the back of the wheel to:
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Match the outer diameter to the wheel's centre bore
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Match the inner diameter to your vehicle's hub lip
Creating a perfect fit between your wheel and vehicle.
Safety Note regarding Spigot Rings: If spigot rings are provided with your wheels, you must use them. You may experience steering wheel vibration, premature tyre wear, and even potential bearing damage without proper centring.
Some wheels are manufactured as "hub-centric," meaning they have a centre bore designed to fit perfectly onto specific vehicles without requiring spigot rings.
The dangers of incorrectly fitted wheels
Choosing wheels with the correct specifications for your vehicle isn't just about aesthetics – it's about safety, performance, and longevity. Incorrectly fitted wheels can lead to:
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Uncomfortable vibrations while driving
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Uneven tyre wear
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Suspension component damage
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Compromised handling and braking
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MOT failures
Find the perfect set of alloy wheels at G Force Styling today
Now that you know what the technical terms mean, hopefully, you feel well-informed about making the right decision for your vehicle. At G Force Styling, we have a wide range of alloy wheels for you to choose from, including a wide array of sizes and styles to match the specifications of your vehicle and your aesthetic needs. If you need more information about a specific set of alloys you have your eye on, you can always contact our team, who will be happy to provide more information.
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