[FChampOH]

My 2 cents if I may:
overall diameter of these various tire sizes.

OE: 255/35/19 overall diameter = 26.00 inches

275/30/19 overall diameter = 25.50 inches

275/35/19 overall diameter = 26.50 inches

About a 1/2 inch difference in O.D. either size from factory. Divide that 1/2 by 2 since it's a round tire and you end up with about a 1/4" difference top and bottom from factory spec. Not really enough to make significant difference in the speedo IMO. Probably would be off be 2-4 mph.

 

[FChampOH]

once again, i say. isnt the speed picked up via wheel speed sensor. aka not in the driveline. so tire size should not matter.

If you change the rolling diameter of the tire, that will affect the speedometer. A wheel speed sensor is reading rotation of the wheel. If you change the height of the tire that will change the rotating speed of the wheel. The size chart I presented was to show that we are splittin' hairs with sizes discussed vs. OE.
In this case, size does matter , but not by much. If you took a sampling of 10 of the same cars from any manufacturer, you would probably find that the mph their speedometers read vary by 1 or 2 mph all things being equal.

The rear end gear swap didn't change the rolling diameter of tires fortunately.