Speedometer is driving me crazy- Something is Wrong

I am driving myself crazy trying to figure out what is wrong and I just can’t.
I purchased a 69 Swinger 340 auto trans a while ago with many small issues that needed to be addressed. One of the issues is the speedometer is way off, 12-13 mph off.
I know the gears have been changed but I don’t know to what.
I did all the calculations I could, rotating the tire and counting the revolutions of the driveshaft 10 times and it always came up more than 3-3/4 revolutions to a little less than 4 revolutions so I guessed 3:73 or 3:91 gears.
I also calculated RPM = MPH x Gear x Gear Ratio x 336 / Tire diameter and using a Department of Transportation Radar unit for the mph. This came out to 3:91.
Anyways I purchased 2 different speedometer gears for the 727 trans using the charts on multiple threads here.
The results are baffling.
Originally my speedometer was off between 13-14mph higher (43mph by radar and 56-57 on dash speedometer)
The original gear in the transmission was a 35 tooth.
I installed a 38 tooth and it corrected 7mph, I thought this is good so I installed the 40 tooth speedometer gear and it changed ZERO results, Nada, no changes, still off 7-8mph.
What am I doing wrong?
And before you ask me “Yes I know how to align the numbers with the teeth when installing the housing”