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I read that the offset for our cars is +42mm, but will rims with +38mm or +40mm fit ?

 
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I can't answer your question about what will fit other than 42. But on many cars, the wheel will not fit if the bolting part is right in the center of the wheel. So an offset basically sets the wheel off-center so it will not interfere with the body of the car or rub.

 
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Offset is the distance (in mm) from the centerline of the rim to the mounting face of the rim. Positive offsets pull the wheel further into the fender well, Negatives (you don't want these) push the rim out. +38 or +40 should be perfectly ok... you'll be pushing the rim out a little further (2 mm isn't even noticable) But don't forget that the rim will also get wider. Offset is related to the rim you are putting on, not the stock one. So a 40 mm offset on a 7" rim is not going to push it as far in as a 6" stock rim. If you're going with a rim that has a lower offset number than stock, take the difference in offsets, convert it to inches (2.54 mm=1") then add it to 1/2 the difference in rim widths

New Rim - Stock Rim
2

That will tell you how much further the rim will stick out. With a 38 offset on a 7" rim, it'll stick out about 5/8" further than stock. You should still be ok... I just measured (very rough guess) but from the face of my stock rim (00 Si) to the farthest point out on the top of the fender there is about 1.5" difference. So it shouldn't stick out.