Unfortunately the majority of alloy wheels aren’t interchangeable between vehicle manufacturers; there are many aspects to finding, choosing and fitting your alloys that you need get correct to ensure the alloys are well suited to your car.
Once you know what to look for its easy to find a good match that will help improve the cars image and performance. Listed below are a few things to look out for before making your decision.
PCD – Pitch Circle Diameter
The correct PCD or Pitch Circle Diameter measurement is essential to make sure you get the correct alloys. If you have the wrong size, the alloys you purchase simply wont fit. The PCD is a measurement system used to attach the wheel against the faceplate and spigot. The most common PCD connection is 4 – 100, meaning that 4 wheel nuts are required to secure the wheels which are spaced evenly around a 100mm circle. The PCD is not always recorded in this format (4-100) and can be commonly found other variations such as 4x100, 4/100, 4100 and some others.
The most common PCD alloy connections contain 4 and 5 studs (but can occasionally go to 8) However the diameter can range from 88 to 130 millimetres in diameter (more in some cases)
Another example of a PCD could be 5-108 but this number will often be unique to you vehicle make and model.
Often Alloy Wheel Manufacturers will provide an alloy wheel in a multi-fit PCD, this means that the alloy will fit both the fitments suggested. e.g. 4-100/108 will fit 4-100 and 4-108 you would use the PCD that matches your vehicle PCD
Vehicle manufactures often use the same measurements within the brand however its best to check your manual if you are unsure of what the measurements are on your vehicle as the PCD varies from vehicle to vehicle. (This information can always be found in your manual, but if you can’t find it contact your vehicle manufacture to be certain)
Spigot Size
The Spigot is located on the connector plate in the centre of the hub of the vehicle, which is the main load bearing connection that the alloy wheel sits on whilst aligning the studs and screwing back the wheel nuts. Generic aftermarket wheels have a larger Spigot hole connection than the spigot of the your vehicle, this is due to vehicles manufactures having different spigots sizes. So by increasing the Spigot size on the alloys the more vehicles the alloys will fit. The Spigot connection must be secure and firm against the wheel, so if the difference is to large spigot locating rings must be used. These are rings of hard plastic or metal that fit onto the cars spigot that allow for a snug connection and ensure the weight is firmly on the spigot and not the studs. It has been known that wheels can break connections under heavy manoeuvring or breaking if the wheel is not resting securely on the spigot.
Wheel Offset
The Wheel off set is taken from the wheels centre line and measured to the mounting face of the alloy wheel; all alloys have a different mounting face, so in turn the offset for each will be different. There are two main offsets Positive and negative. The majority of modern vehicles have a positive offset, and older vehicles and SUV’s tend to have a negative offset, it is important that you ensure the wheel and tyre is going to be clear of any bodywork, suspension, steering and braking components. It’s usually accepted that you can go 5-7mm outside the recommended vehicle guidelines although the further you go past the recommenced allowance the more the performance will be affected.
Positive Offset - the mounting face of the wheel is forward of the centre line meaning you have to push the alloy further into the wheel arch before you can attach the wheel mounting face to the Spigot, therefore bringing the wheel further under the bodywork of the vehicle to connect.
Negative offset – The mounting face of the wheel is behind the centre line so when connected to the spigot the faceplate will have less distance under the wheel arch and the wheel will be pushed outwards from the vehicle bodywork.
Aspect Ratio
The aspect ratio refers to the ratio between the tyres width and height of the side wall, this information is always located on the tyre, and refers to the height of the tyre wall being a percentage of the tyre width. E.G 195/65 – meaning the width is 195mm and the height is 65% of the 195mm in this case 126mm.
The aspect ratio is calculated using this formula: (Height / Width * 100)
OR to work out the Aspect: (Width * Percentage / 100)
It’s usually considered that lower aspect tyres provide better handling while larger aspect tyres provide a more comfortable drive.
Plus Sizing & Up Stepping
Plus Sizing and Up Stepping is done to maximise the cars performance and also allows you to up the size of the alloys. When changing your alloys you can either upgrade the for the same size alloys and keep the same size tyres, or move up 1” or 2” (e.g. 15” to 16 or 17”) and reduce the size of the tyre. This is done by using a lower profile tyre (increasing the tyre width and reducing the tyre height). This will give the vehicle a different handling characteristic being much more stable and responsive when cornering, however the down side to this is that ride comfort is reduced making the bumps and divots more noticeable.
When Plus sizing its important to stay as close to the original tyre specification as possible, it’s usually acknowledged that a 3% difference acceptable. This is important because the larger the difference the more likely you will encounter problems with the gear shift points, speedometer (travelling faster than you think) fuel consumption, braking systems and onboard computers.
How Plus Sizing Works
To get a tyre combination to the same as before (Factory Standard)
Your Factory or OE (original equipment) tyres have all the details on you need to allow you to calculate the information.
Example shown – 205/50 R 16 80 V
205mm Wheel width, 50 height in percentage (50% of 205mm), R = radius, 16 = Wheel Size, 80 = pressure & V = Speed rating (more information below)
First take the tyre size, this is the width of the tyre, in this example is 205, the next number is the height of the tyre wall in a percentage, the height of the current tyre is 50% of 205 = 102.5mm. this can be worked out in this calculation, 205*50/100 = 102.5 (obviously change the width and height numbers accordingly)
Multiply the figure by 2 so you get the top and bottom wall and you will have the total height of the tyre in mm (in this case it’s the same as the width, but not always), 102.5 x 2 =205mm
Divide this by 25.4 to calculate inches this example = 8”
So now we know that you have a 16” wheel (in the example above) with tyre height of 8” so a total size of 24”
You can use the same calculation on the new tyres to ensure a good match
Another example 215/45 R 17 80 V
Tyre height = (45% of 215) 96.75 * 2 = 193.5 / 25.4 = 7.6” +17 =24.6” total alloy & wheel height
Now you know how to match you new wheel – tyre combination to get a similar measurement as the factory fitted wheel.
Another rough guide to do this is to;
- Increase the width by 10mm
- Decrease the aspect (height) by 10 points
- Increase the alloy diameter by 1”
Or
- Increase the width by 20mm
- Decrease the aspect (height) by 20 points
- Increase the alloy diameter by 2”
Increased Width
If you are going to plus sized and increase width of the wheel, try to increase the width in even proportions equal to either side of the mounting face plate. If the offset is the same or similar to the old one this will not be a problem, however if you have an alloy that has a larger offset to the factory wheels the balance will be out. This is because the wheel connection is further forward or behind the original factory wheel connection and you have and the width of the tyre is also increased.
For example if you have to move the connection plate further forward by 10mm from an offset of 38 to 48 and have added 10mm onto the tyre, the outer line of the wheel will be 15mm out of place and change the tracking. It is not always possible to achieve the same ratio but try to avoid excessive changes.
Speed Symbol
All tyres carry a speed symbol in the form of a letter indicating the maximum speed for which the tyre is intended to be used. This chart shows which speed is assigned to each letter.
N = 87 MPH
P = 93 MPH
Q = 99 MPH
R = 106 MPH
S = 112 MPH
T = 118 MPH
H = 130 MPH
V = 149 MPH
W = 168 MPH
Y = 186 MPH
ZR = 149 MPH
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