Okay I'll explain this leaving out as much detail as I can, because it can get quite boring and quite technical when talking about centre bores etc.
PCD
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The PCD is the single most important thing when buying a set of wheels. If the PCD is wrong, the wheels won't physically go on the car no matter how much you kick them. The PCD is basically the stud pattern. The measurement measures how far the holes on the wheels are away from each other. So a PCD of 4x100 means there are 4 bolts with 100mm between them. Incidentally, 4x100 is the most common wheel measurement. Fiats (and some alfa's and lancia's) use a fairly rare PCD of 4x98. You can get 4x100 wheels to fit with wobbly bolts, as 2mm isnt that much. Personally, if at all possible I would try not to use them. Having said that people who have used them don't report any major problems, other than the occasional wheel vibration over 80mph. (This could be in their head or even wheel balancing - it probably isnt even the bolts). Remember though that you'll have to buy the bolts adding £25 or so onto the cost of your alloys.
OFFSET
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The wheel offset is less vital to get right than the PCD, but still important, especially if you are fitting alloys that are much bigger than your car's standard wheels. Its the distance from the mounting part of the wheel (the bit that touches the hub) to the actual centerline of the alloy wheel. The higher the offset, the further "in" to the car the wheel will sit, the lower the offset the further "out" the wheel will sit. The problem is that when you buy larger and wider wheels than standard, you don't necessarily need the same offset as the standard wheel was. This makes things difficult when chosing the offset for your car. As a general rule, aim for as close to the standard offset as possible if youre not sure. (or ask someone who knows). I'll give you two examples of how you can cock this up. First, on a seicento if you chose an offset that is too high, like ET40 (ET means offset), then the wheel will rub on the inner trailing arm, particularly if you have bought wider wheels than standard. So when fitting 6 inch wide wheels try to get an offset as close to ET30 as possible. Because this is difficult, and the most common offset is ET35, you'll probably fall in love with some ET35 wheels. In this case, you can fit wheel spacers that "push" the wheel further out. An easy (but technically innacurate) way to think about it is that a 3mm spacer reduces your offset by 3mm. So a 3mm spacer on a ET35 wheels makes them ET32, and now they should fit okay. The second way you could cock it up is to buy an offset that is too low. This will push the wheels out towards the arch too much. In a minor case you can do some arch work and they shouldnt rub. In a major case, they will stick out of the side of the car like a beach buggy. My Dad demonstrated this nicely when he fitted 550 Maranello wheels to his Testarossa. The wheels were not only slightly wider than standard, but the offset was far too low. The rear wheels stick out of the arch by a good inch, and it looks ridiculous. Unfortunately, whilst you can fix a high offset problem with spacers, the only way to fix a low offset problem is to machine the wheel, which is expensive and has safety implications.
So there you go - PCD and Offsets explained, probably as clear as mud.
and kind of understood it. Let me try :
