Speedometer reading low

[lsupcar]

My speedometer is reading low. When the speedometer reads 30, GPS says 40, speedo 50, gps 60ish, speedo 60, gps 73ish, speedo 20, gps 25ish.
Yes, new tires , stock sized Michelin 225/50/16 on stock rims. I swapped back to old aftermarket 16 rims and the old original Bridgestone potenza RE95's in 225/50/16 earlier today, and the speedo remained off the same way. Both sets of tires were also roughly the same size, taking into account newer tread on the new tires. I also measured the new tire's revolutions per mile, and it measured out within specs at about 836.
Speed sensor 2 in place with no visible problems. No CEL or irratic speedo operation. At idle and at 3000 rpm the alternator reads 13.7 to 13.88 volts.
Any ideas?
Can't find this issue in a search.
BTW '92 with 155K miles.
Thanks.
Bill Z