You are right on with your concern re: mismatched tires on AWD. Particularly with the first series Volvo XC's, you can only have a minimal difference in tread depth, tread type, even brand of tires. It appears to be the single biggest risk in these AWD systems, and has caused many thousands of dollars in repairs. We lost one tire out of a set of Michelins that had 8/32nds of tread left (out of original 10/32nds) and Volvo USA formally advised that we had to buy all four new tires to avoid damage. I believe thaqt they now say that there can be no more than about 2/32nds, and that the deeper tread must always be on teh front axle if there is any difference. This is true for teh whole range from'98 -'02. New Haldex system appeared in '03 and is apparently slightly less at risk.
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Mike Sullivan '07 XC70, '95 855T (100k), '95 855 NA (148k). Past Volvo's: '85 744 (256k), '86 245 (165k), '86 245 (195k), '88 745 (208k), '91 745 (196k), '93 965 (147k) , '98 V70XC (98k).