Maybe I can add to the confusion. :0

I am fairly certain, based on period road tests and the green shop manual, as well as having owned a couple near-new 544's, all US 544's came on 5.90-15 bias ply tires. These are the same diameter (~25.4") as a 165R15, which BTW is an 80 series tire (165R80-15), not a 70 or a 78.

At typical highway speeds, the bias ply tires actually grew a few percent in diameter, so the speedometer was usually more accurate with bias ply tires than with radials. This is due to the fact that typically, the speedo in mid-60's Volvo sedans with stock bias ply tires and stock gearing read 3-4% optimistic at highway speeds. If you put 165R15 radials on that car with no other changes, the discrepancy became worse, because the steel belted radial did not 'grow' at speed.

Gary L
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1971 142E ITB racer, 1973 1800ES, 2002 S60 T5