I have to agree with klaus, just buy another of the same rim that's on your other 4 wheels. The 15 and 16 inch wheel/tires will fit in the well, if you throw out the gray shallow tray that sits over the spare. Ther may be some clearance problems with the 17 inch wheels. A full-size spare was an option on Volvo factory euro-delivery cars, I think a full-size spare is mandated in certain countries, like Australia.

-BTC

With all of the extra wheels on Ebay, buy one that kind of look like it fits on your car and throw the spare rim away.

Klaus
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98 V70Rawd(101Kmi), 95 854T(85K mi), 75 164E(173K mi)

The only concern is the width of the spare rim as compared to the width of the regular wheels. All tires are designed to be mounted on a limited range of rim widths depending on the width of the tire. If you mounted a wide tire on a narrow width rim, you could be creating a safety problem in that the tire bead may not be sitting properly on the rim flange. You may also create a handling problem with this conbination.
If this is the case, your donut spare tire on the spare rim would actually be a safer alternative.
For my 740, I found a "narrow" tire that had the same diameter as the regular tires and mounted it on the narrow spare wheel. This way the tire fits the rim properly and it will still carry the load.