I recommend that you get a set of Volvo wheel covers. They do sell an inexpensive set, as well as some better-looking, more expensive ones.

I once used a cheap set from JC Whitney, and although I liked the cover's looks, my lug nuts got all rusty. Seems to me that generic wheel covers must fit a variety of wheels, so they do not fit close to the wheel. The cheap wheel covers also use metal clips that scratch all the paint off your wheels.

The Volvo wheel covers that I own have a ring on the back of them that fits snug against their wheels, keeping crud out.
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96 855R, 95 855,854, 90 744 Ti - 340,000 Volvo miles put on 7 bricks