I dont understand this concept. Is more offset (larger mm) better or worse than going the other way. Is a 42mm better or worse than a 38 mm offset on 17" tires.

first of all offset doesnt apply to tires only rims, so if your asking what off set for a 17" rim then it depends on what suspension setup you have, for standard ride height then 38mm off set will do, if you have your car lowered then 43+mm is required so as the wheels will clear the inner arch if you hit a large bump/pothole or have a full load on in the back. Offset expressed in mm is the distance from the centre of the wheel section to the mounting face. The larger the figure the further the wheel will sit into the arch, so therefore a 38mm off set wheel sits out further, and can cause clearance problems on lowered cars.

Offset is a measure of the distance from the center of the wheel (thickness-wise) to the hub-attachment-point. A negative offset means the hub attaches to the wheel at a point that's more toward the back of the wheel from the center line. A positive offset means the hub attaches to the wheel at a point that's more toward the front (face) of the wheel from the center line. For a 17x7 wheel, an offset between +40 to +45 is recommend, with best being closer to +45 (e.g. volks use +44, just about perfect). If you go lower (+38) you risk rubbing the wheel well linder or fender. If you go higher, you risk rubbing suspension components. The correct offset depends on the car it will go on and the wheel width. If you need more info, there should be topics in the various forums and in the newbie forum that may have more detailed, or just better...I may not make sense now as I'm getting tired from work, info so try searching in those. HTH.

The sole reason that you need to increase offset is because almost all aftermarket wheels are wider than stock. If you lower your car w/ stock wheels (like me), most likely you won't have any problems (like me). If you put on wider wheels w/ stock suspension, you may or may not have problems. If you do then you need a higher offset. If you put on wider wheels AND lower the car then you almost certainly need a higher offset to clear the fender.

On my old car, I went from 14x5 w/ 185/70-14 to 16x7 w/ 205/50-16, but I kept the same 35mm offset as stock. The front tires stuck out past the fender about 1/8" but to my knowledge it never came into contact w/ the fender because it wasn't low enough (before lowering), or my new springs and struts were stiff enough to keep it from contacting the fender. My rear tires scraped the fender lip even before lowering, if I had someone in the back and made a turn. I bent the fender lips up and cured that problem. If my car was lowered any more in front, the tires would have smashed into the fenders. Well anyway, hope this helps some.