'95 speedometer reading incorrectly slower than actual

[92Raiderburban]

I have a squarebody suburban that I swapped a '95 cluster + VSSB into. I have a '93 4L80 (toggle switch manual shift) behind its current carbed powerplant. This is the connections: Cluster plug ----> VSSB ----> 4L80 output sensor. There is no TCM/ECM/etc. I have the tall, factory rare 3.21s out back with 295/75/16 tires (33.42 roughly) front is factory 265s. I use the calculator on tbi chips.com and the input ratio i got was 0.605xxxx, closest to to the chart setting 0.604xxxx, I calibrated the jumpers on the VSSB and it reads 5 MPH slower. i compared with my GPS phone app. I tried these jumper settings also but no luck to get it right:

0.591xxxx - 7-8 mph slower
0.598xxxx - 5 mph slower
0.611xxx - 0-20 dead on, at 25 mph read 5 mph slower
0.618xxx - read 7 mph too fast

What could be the culprit? What could i do to get it reading correct??

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[92Raiderburban]

UPDATE!! Last night I went up to the 0.625 input and again 5 mph slower. Went up again to the 0.632 input and its dead on all the way thru matching my GPS speedo. KEEP IN MIND for those who are doing taller gear swaps and keeping bigger than factory tires, that the calibration calculator is not always accurate. Try different ones one on the chart whether its a lower than your input **OR** higher.

In my case:

0.591xxxx - 7-8 mph slower
0.598xxxx - 5 mph slower
*0.604xxx - closest to my calculated input (3.21s and 33.42 inch tires--> 0.605) read 5 mph slower than GPS
0.611xxx - 0-20 dead on, at 25 mph went 5 mph slower
0.618xxx - read 7 mph too fast
0.625xxx - read 5 mph slower
*0.632xxx - correct reading, dead on with GPS and school zone speed sensor.

Now at 0.632xxx, the CALCULATOR specs are off on either rear gear or tire size. 32.24 inch tires/3.21's ***OR*** 33.42 inch tires/3.35s. I know damn well I have the factory 3.21's out back, so just a warning for those who get the calculator giving ratios that are off compared to your trucks specs. Keep trying them and mark em down 'till you get it right.