Whenever I see people on here talking about wheels, as well as the wheel rim width and diameter they mention another number that appears to be important... ET? I gather, I think, it is the offset of the inner face of the hub mounting... but I'm not sure. Could someone please explain? Also, then, when looking at wheels for my car, how can I possibly know what this number needs to be? My car is currently fitted with 8.5J x 18 fronts and 9.5J x 18 rears (what does the J stand for?). The car when new would have been standard with 8.5J x 17's all round. If I decided to fit 17 inch wheels for winter, same size all round, what would this third mystical number be? The offset one? Then, knowing that, am I restricted to wheels designed specifically for the SL or would 8.5J x 17 inch wheels from other models be identical? Sorry, lot's of questions, but it's an area I find quite mind boggling when you see how many wheels are available and one I don't understand at all. Thank you.