What does Offset mean?

The distance between the centreline of the wheel and the plane of the hub-mounting surface of the wheel is known as the wheel offset.

The offset can be either positive or negative, and is typically measured in millimeters. Offset has a significant effect on many elements of a vehicle’s suspension, including suspension geometry, clearance between the tire and suspension elements, the scrub radius of the steering system, and visually, the width of the wheel faces relative to the car’s bodywork.

Offset is usually stamped or engraved into the wheel and is measured in millimetres of ‘ET’ [ET is the short form of the German word 'Einpresstiefe' which literally translates as 'insertion depth']An example would be “ET45″ for a 45mm offset.

Zero Offset wheels have their mounting face even with the centreline of the wheel and are by definition “ET 0″.

Positive Offset wheels have their mounting face toward the front face of the wheel.

Negative Offset wheels have their mounting face toward the rear of the wheel.

The easiest way calculate wheel offset is as follows:

First, measure the overall width of the wheel (remember, just because a wheel is 18×7.5, does not mean that the OVERALL width is 7.5”. It means that the measurement between the outboard flange and the inboard flange is 7.5”). Next, divide that width of the wheel by two; this will give you the centreline of the wheel.

Overall width/2 = Centreline

After determining the centreline, measure from the hub-mounting surface of the hub to the edge of the inboard flange (if the wheel were laying flat on the ground – face up – your measurement would be from the ground to the hub-mounting surface). This is your back spacing.

Back spacing – Centerline = Offset

What does PCD mean?

The Pitch Circle Diameter (PCD) is the diameter of the circle which passes through the centre of all the studs, wheel bolts or wheel rim holes.

The easiest way calculate the PCD is as follows:

1) Identify the size of rim or tyre size. (This helps to narrow down the likely PCD sizes)
2) Measure the distance ‘S’ between two adjacent studs from the centre of each hole.
3) Calculate from the formula below

PCD Calculation Formula

4 Stud PCD =     S / 0.7071

5 Stud PCD =     S / 0.5878

6 Stud PCD =     S / 0.5

What does Centrebore mean?

The ‘centerbore’ of an alloy wheel is the size of the hole at the back of the wheel which the ‘hub’ fits into. To help the wheels to seat properly this hole needs to be an exact match to the size of the hub.

Most modern wheels are what’s called ‘hub-centric’ – this means that the hub which protrudes from your car [and mates with the equivalent sized hole at the back of your wheel] is ‘load bearing’. All the studs or bolts do therefore is hold the wheel onto the hub!

If you have’ lug-centric’ wheels, the state of your studs or bolts is obviously more critical – be sure to replace these from time to time and always 3/4 tighten the wheels off the car to ensure they’re centred.

What does Plus-Sizing or Up-Stepping mean?

Plus-Sizing or Up-Stepping are two terms given to the practice of increasing the diameter of your wheels whilst simultaneously reducing the profile of your tyres to keep the overall rolling radius the same.

Benefits - Plus-Sizing will improve the handling of your car! – each step will reduce the proportion of flexible tyre ’sidewall’ to rigid alloy. This will improve response, will help keep the tyre tread square to the road and will improve your car’s ‘feedback’. If done properly speedo and odometer accuracy will be retained and the car’s sure to look better..

Disadvantages - In the majority of situations, tyre inches are lighter than wheel inches. Plus-sizing can make your overall wheel/tyre package heavier. Reducing the profile of your tyres will also reduce your car’s damping deflection under compression [the ride quality will get worse] Other disadvantages can include you needing more expensive tyres, your brakes looking puny and people’s grannies laughing and calling your car a pram.