Measure & Identify Alloy Wheel Sizes

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Diameter Measurement
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Diameter Measurement

Diameter - How Big Are They...?

The diameter of a wheel can be found by measuring across the face of a given alloy wheel (excluding the lip flange) making sure to pass directly through the centre of the wheel. You need to compensate for the mounting flange as they are not included in the measurement.
Width Measurement
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Width Measurement

Width- How Wide Are They...?

The width of a wheel can be found by measuring across the wheel from front to back (Also called the “Depth”) Again the mounting flange is to be ignored and just a measurement of the inner barrel is to be taken to determine the true width of an alloy wheel.

Adding wider wheels increases a vehicles track and can aid handling if also equipped with slightly wider tyres than standard. Each car has a certain range of alloy wheel widths that can be fitted safely. Go too wide and you’ll most likely end up with some sort of scraping, either on the inner strut or outer arch. Both potentially dangerous situations if it causes a tyre blowout at speed!
Stamped Sizes
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Stamped Sizes

Lucky Numbers

A much easier way than physical measuring is to check the back of the alloy wheel. Most manufactures have the diameter, width and offset stamped on the back of the spokes upon production. 

If you can't see any numbers, give your wheels a clean as the digits could be obscured by years of dirt, grime and accumulated brake dust. Then punish yourself for letting your wheels get into that state in the first place... Haven't you heard of Voodoo Ride...?!?!? 8-)
Offset (ET) Marking
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Offset (ET) Marking

Offset - Also Referred to as "ET"

Manually Calculating a wheel's offset is a good number crunching exercise but for the sake of this guide we will stick to looking on the back of an alloy wheel to see if we get lucky and the offset is indeed stamped on the back of the wheel, as is the case with most aftermarket alloy wheels.

Offset - Just What Do Those Numbers Mean...?

Offset is a term used for the measurement of an alloy wheel and how far in, or out, it sits in relation to the mounting face of the hub. If you imagine a line running down the centre of an alloy wheel, and the wheel mounting face sat directly on this imaginary line, then you would call that a +0mm (Plus Zero Millimetre) offset wheel.

Any further forward to the face of the wheel becomes positive offset as the mounting face on the hub moves closer to the face of the wheel. If, for instance, the hub mounting face was 40mm in front of the centreline then you would call that a +40mm (Plus Forty Millimetre) offset wheel. Positive offset wheels are easily identified as they have shallow spokes on the face and a larger inner wheel barrel.

On the other hand, should the hub mounting face be behind the centre line, then that can be said to have a minus offset. As another example, if you imagine the hub mounting face was 40mm behind the centre line then you would call that a -40mm (Minus Forty Millimetre) offset wheel. Negative offset wheels are easily identified as they either have very deep concaved spokes, or real deep front face dishes and a smaller inner barrel compared to a positive offset alloy wheel.

All cars require a unique offset range. This is where the outside edge of the wheel sits in relation to the arch of the car. It is possible to go roughly 5-10mm outside these recommendations, but always consult a specialist if you are unsure, as there are often other factors that need taken into consideration, such as wheel width.

As a general rule, using a lower offset than standard will help to fill out the arches on a given car. But be careful, if you go for too low an offset the wheel and tyre may rub on the vehicles arches. The same can be said for using too high an offset, as in that case the wheel can sit too far in the arches and could end up scrubbing on the suspension components.
Size, Width & PCD Lucky Numbers
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Size, Width & PCD Lucky Numbers

PCD - The Most Confusing Measurement...

PCD stands for (pitch circle diameter) and is the diameter of a circle drawn through the centre of the wheel bolt holes. PCD is measured in millimetres and also indicates the number of studs or bolts the wheel has.

The most common fitment has 4 studs and a Pitch Circle Diameter (PCD) of 100mm giving the fitment “4x100”. The most accurate way to measure PCD is with a PCD guide, but if you do not have one available it is possible to measure yourself roughly with a ruler.

With a 4 stud wheel, simply measure the distance between the centres of 2 opposing wheel bolt holes across the centre of the alloy wheel. Measuring a 5 stud wheel is trickier as you cannot simply measure between 2 opposing wheel bolt hole centres. Sometimes wheel PCD’s, Widths and Offsets are stamped on the inside of the alloy wheel on the back of the spokes. So it’s easier to look there first before trying to measure yourself as some fitments are very close to each other, ie 4x98 & 4x100.
4x100 Single PCD Alloy Wheel
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4x100 Single PCD Alloy Wheel
4x100 & 4x114.3 Duel PCD Alloy Wheel
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4x100 & 4x114.3 Duel PCD Alloy Wheel
Centre Bore Measurement
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Centre Bore Measurement

Centre Bore - Yet More Numbers...?

The Centre Bore Measurement is simply achieved by measuring across the centre hole diameter on the back of the Alloy Wheel. Sometimes you can be surprised by the Centre Bore Lucky Number on the back of the Alloy Wheel. This one didn't have it, so we measured it and ended up with 67.1mm CB.

To make matters more confusing, just because a wheel from one vehicle has the same PCD and offset as a wheel from another vehicle, it doesn’t mean that they are interchangeable!

The Alloy Wheel Centre Bore and the Vehicle Hub Bore must also match so the wheel sits perfectly centred on the hub to avoid any vibrations at speed. This is achieved by using Spigot Rings. We cover the whole topic of Spigot Rings in our other guide: Spigot Rings - Just What Are They...?

Legal Mumbo Jumbo

Funky Rims make no representations or warranties of any kind, either express or implied, as to the completeness, accuracy, reliability, suitability or availability of this Guide or the information, products or services contained in these pages or their fitness for any purpose whatsoever. Any reliance you place on such information is therefore entirely at your own risk.
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