Speedometer correction - Saginaw 4 speed

[Mbeef61]

I was out today doing some math to find my transmission gear ratios. I have a 71 stepside 350 with saginaw 4 speed. 3.73 gears 295/50/r15 (26.6") rears. I am trying to correct the speedometer and am not sure how to go about it. I have tried to search but cannot make sense of it.

Turns out it is a "2-line" saginaw 4 speed with gear ratios as follows:

1 - 3.11
2 - 2.20
3 - 1.47
4 - 1

My speedo compared to GPS is 1.3 too fast.

For example:
Speedo says 13, GPS 10
Speedo 39, GPS 30
Speedo 65, GPS 50
Speedo 95, GPS 70

How do I go about correcting this?