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A beginner's guide to wheel sizes

A beginner's guide to wheel sizes

Not all wheels fit all cars, and if you're considering fitting new wheels, then you'll need to read our handy guide which tells you all you need to know!

Diameter

This is always referred to in inches, and as the name suggests is the diameter of the wheel – not the tyre and wheel together.

Regular road cars generally have wheels that are between 14 and 19inches in diameter, with newer, more sporty, and luxury 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 diameter, with the smaller number relating to the width.

Width

Again this is measured in inches. The width of the wheel dictates what tyres you can fit to the car – The wider the wheel, the wider the tyre and so the more tyre is in contact with the road.

This means there's more available grip and so the car will have better roadholding and braking than an identical car with narrower tyres.

Aftermarket Wheels

In general if you intend to keep the wheels on your car, or maybe even upgrade to ones of a model higher up the same range then you don't really need to concern yourself with any more wheel measurements.

If however 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. Fit a wheel with too little, or too much offset and it will either stick too far out, or too far in the arch, and risk fouling on the bodywork or suspension components.

How offset is actually measured can be a little confusing, but the measurement refers to the distance from an imaginary line drawn exactly through the centre of the wheel to the mounting face of the wheel where it bolts against the brake disc.

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. Got that?!

In general you need to stick to a very similar offset if you're fitting new wheels. If you stray too far outside of this measurement you'll find the wheels don't fit, will catch on the bodywork or suspension, and probably look really silly!

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 to keep it simple always keep it the same!

Rolling Radius

If you fit different wheels/tyres to your car it could affect the rolling radius. This is the circumference of the tyre. A larger tyre will have a larger rolling radius, and as such will affect gearing, and the accuracy of your speedometer.

If you fit larger wheels you need to fit lower profile tyres (tyres with a smaller sidewall) to maintain the same rolling radius. Google 'Tyre size calculator' to work out what size tyres you need!

A beginner's guide to wheel sizes