I believe that any of the OEM wheels for front drive Volvos will fit, ie: 850, S/V70, S60 and S80. The 850 was sold with 15, 16 and 17 inch wheels by factory, and it's just a matter of getting the right tire size to go with the rim diameter you choose in order for them to fit. Note that the 17's on the 850R's generated lots of complaints of very harsh ride, so much that Volvo offered to swap in the 16's at no charge. All of the high pressure turbo's came with 16's I believe (except 850R), with a size 205/50-16 tire. If you go to a 205/55-16, there have been some number of complaints of tires rubbing at full lock (see pics of such on Bay 13), but the 205/50 size has pretty thin number of choices so some move up to the /55's for better selsection and better price. The 15's take 185/65-15, 195/60-15 and even 205/55-15 sizes, although I don't think that Volvo ever offered the 205/55-15 as a factory fitted size. REmember that the bigger the rim, the harsher the ride is going to be, since it forces you to buy tire with a narrower sidewall height.
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Mike Sullivan ('91 745 (171k), '93 965 (147k), '95 855T (60k), '98 V70XC (78k). Past Volvo's: '85 744 (256k), '86 245 (165k), '86 245 (195k), '88 745 (208k).