The term Offset or the abbreviation ET for “Einpresstiefe” in German (literally, “insertion depth”) refers to the distance the mounting face of the wheel is away from the centre line of the wheel.
The hub mounting surface is at the centreline of the wheel.
The hub mounting surface is toward the front or wheel side of the wheel. This is what we find on the RS and all recent Porsches and has much to do with the way the suspension and ABS interact.
The hub mounting surface is toward the back or inner side of the wheels centreline. The wider Fuchs wheels are typically a negative offset.
If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset should also changes numerically. If the offset remains the same while you increase the width, the additional width would be split evenly between the inside and outside of the wheel and for most cars, this won’t work correctly.
For any given width of wheel, reducing the offset moves the rim towards the wheel arch, (this is the effect achieved by fitting spacers ).
There is an excellent calculator at http://www.1010tires.com/WheelOffsetCalculator.asp
Get it wrong and you may not be able to fit the wheels at all, which could be both aggravating and expensive, there are a number of Porsche wheels styles which use a wide variety of offsets, not all are interchangeable.
You should also bear in mind the offset of the wheel is a factor in the load put on the wheel bearings, which may cause them to wear prematurely – and in extremis fail, and although three wheeling round a circuit may save on tyres it can be bad for your health and won’t endear you to the clerk of the course or a trackday organiser.
In fact not just the wheel bearing can be subject to extra loads, other steering and suspension components can be subject to greater stresses, and this is further compounded by using slicks or track tyres where the levels of grip impose more load than standard.