WHEEL OFFSET, PCD, & CENTRE SPIGOT DIAMETER
Offset is the distance between the imaginary centreline of the wheel (as viewed from behind the wheel as it would roll away from you) and the inside face that bolts up against the wheel hub on the car. Fitting wheels that alter the track of your car (the distance between the centre of the left and right wheels, either front or rear) by more than 20mm will usually void any insurance policy, that means the offset cannot be any more than 10mm different per wheel than standard.
The other two things to look for when fitting after market wheels is the PCD (Pitch Circle Diameter, ) and spigot size. The PCD is easy to match as this relates to the number of studs you need to hold the wheel on the car. The ACTUAL meaning is the diameter of the studs from the centre of the wheel.
4 HOLE WHEELS Measure the distance between the centres of 2 OPPOSITE holes OR Measure the distance between the centres of 2 ADJACENT holes and multiply by 1.414
5 HOLE WHEELS Measure the distance X between the centres of 2 ADJACENT holes and multiply by 1.7012
Spigot size is a bit trickier....the spigot is the bit in the centre of the hub that you rest the inside centre of the wheel on whilst aligning the studs and screwing back the wheel nuts. On generic after market wheels, the spigot hole inside the wheels is a lot bigger than the spigot on the car. So what you need to do in this case is fit spigot locating rings. These are just rings of aluminium or hard plastic, that fit over the spigot on your car and then have a proper snug fit with the spigot hole on the wheel. If you don't have the spigot taking all the weight of the car, chances are you'll break one or more studs when you drive the car hard or have to brake hard. The wheel nuts are simply there to hold the wheel on, NOT support the weight of the car. Also, as there is nothing to centre the wheel, you'll notice the wheels go in and out of balance because as you drive around, they'll move around on the hub. Of course, this is not something that those of us with centre-splined wheels need worry about.