My family had two volvos that behaved like this. One was an '81 Volvo 240 DL manual, and the other was a '92 Volvo 740 automatic. Both of them could get up to highway speed noticeably faster if you turned off the A/C. (My dad used to say they’d “do 0 to 60 on a leisurely afternoon.”) There’s some chance that A/C was aftermarket on the '81, so maybe that’s a factor? But the '92 did it too, and that was definitely stock.