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A beginner's guide to wheel sizes

A beginner's guide to wheel sizes

Are you thinking about replacing the wheels on your car or van? If so, you need to make sure the rims you buy fit properly. Our handy guide tells you all you need to know.

Diameter

This is the diameter of the wheel on its own, not the tyre and wheel together. And it's always measured in inches.

Regular road cars generally have wheels that are between 14 and 21 inches in diameter, with newer, sportier, and more luxurious cars generally having the larger sizes.

Where you see the wheel size written down such as 7x17in the larger of the two measurements will virtually always be the diameter, with the smaller number relating to the width of the wheel.

Width

Again this is measured in inches. The wider the wheel, the wider the tyre and, in theory at least, the greater the grip.

This means the car should have better roadholding and braking than an identical car with narrower tyres.

Aftermarket wheels

If you're considering fitting aftermarket wheels then it's worth familiarising yourself with the following measurements.

Offset

The offset is the measurement that dictates how far your wheels stick out from the car's wheelarch, or indeed, how far they're recessed into the wheelarch. Get this wrong and your wheel can either stick out beyond the wheelarch, rendering it illegal, or the inside of the tyre can rub against either the inside of the wheelarch or even worse, suspension components.

So how do you measure offset? Well, start by standing the wheel up vertically, and drawing an imaginary line round the centre of the wheel. Then stick a ruler or measuring tape horizontally through the centre of the wheel face (where the hub sits when the wheel is mounted on the car). Then measure the distance between the wheel's centreline and the back of the mounting face of the wheel (in essence, the backside of the bit the wheel bolts go through).

Offset is expressed in mm using the term ET (this is derived from the German word 'Einpresstiefe' which means 'insertion depth').

So if you see a marking on your wheel which says ET30, you know you have wheels where the hub face is 30mm further outboard from the dead centre of the width of the wheel.

An ET of 0 means that the mounting face is exactly in the centre of the wheel, and a ET of -10 mean that the mounting face is 10mm further inboard than the centre of the wheel.

Centre bore

The diameter of the hole in the middle of the wheel is the centre bore. It's usually covered by a centre cap – but pop the cap out and you'll see the hole.

This fits over a matching diameter flange on the hub face. If the centre bore of your wheels are smaller than that of your car the wheels simply won't fit.

If the bore is larger on the wheel then you'll need what are known as spigot rings. These are effectively large plastic seating rings which allow the wheel to be correctly centred on the hub.

PCD

This stands for Pitch Circle Diameter and refers to the number of wheel bolts and the diameter of an imaginary circle if it were drawn through the middle of the bolts.

So if it's a 5x104 PCD there are 5 wheel bolts in a 104mm diameter circle. If you're considering swapping wheels, then you must get an identical PCD.

There are various options with wheel hub adaptors and what are called 'wobble bolts' if you really must fit wheels with a different PCD – but it's a lot easier just to keep it the same!

Rolling radius

The rolling radius is the circumference of the tyre. A larger tyre will have a longer rolling radius, which will affect not only the car's gearing, but also means your speedometer will read incorrectly.

So, if you fit larger wheels you need to fit lower-profile tyres (tyres with a shallower sidewall) to maintain the same rolling radius. Google 'tyre size calculator' to work out what size of tyres you need.

A beginner's guide to wheel sizes
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