VSSB DRAC Calibration

Some 1991 and all 1992-1995 trucks calibrate the speedometer using either a DRAC or VSSB module.  Both look VERY similar.  The early DRAC used a 7 pin layout and the 1994-1995 VSSB used 14 pins.  You can use a 14 pin VSSB as a replacement for a DRAC but not the other way around.

DRAC VSSBThe way I recalibrate these is to just MOVE the jumper bars to the newly calculated positions for your gear ratio and tire size.  The old site done by someone else had you installing DIP switches which would have been nice had GM done that originally but since not something you change that often.  I find it easier to just move the jumper bars and certainly cheaper.  I am trying out a new calculator on this page to see if I cant make things easier for everyone.

 

 

Jumper Settings Set to closest value

1 = Jumper connected or switch ON
0 = Jumper NOT connected or switch OFF
1-7 = Jumper/switch #
Input Ratio 1 2 3 4 5 6 7
0.500000 1 1 1 1 1 1 0
0.505958 1 1 1 1 1 0 1
0.511230 1 1 1 1 1 0 0
0.517090 1 1 1 1 0 1 1
0.522949 1 1 1 1 0 1 0
0.528809 1 1 1 1 0 0 1
0.534668 1 1 1 1 0 0 0
0.540527 1 1 1 0 1 1 1
0.546875 1 1 1 0 1 1 0
0.552734 1 1 1 0 1 0 1
0.559082 1 1 1 0 1 0 0
0.565430 1 1 1 0 0 1 1
0.571777 1 1 1 0 0 1 0
0.578125 1 1 1 0 0 0 1
0.584473 1 1 1 0 0 0 0
0.591309 1 1 0 1 1 1 1
0.598145 1 1 0 1 1 1 0
0.604492 1 1 0 1 1 0 1
0.611328 1 1 0 1 1 0 0
0.618164 1 1 0 1 0 1 1
0.625448 1 1 0 1 0 1 0
0.632324 1 1 0 1 0 0 1
0.639648 1 1 0 1 0 0 0
0.646484 1 1 0 0 1 1 1
0.653809 1 1 0 0 1 1 0
0.661133 1 1 0 0 1 0 1
0.668457 1 1 0 0 1 0 0
0.676270 1 1 0 0 0 1 1
0.683594 1 1 0 0 0 1 0
0.691406 1 1 0 0 0 0 1
0.699219 1 1 0 0 0 0 0
0.707031 1 0 1 1 1 1 1
0.714844 1 0 1 1 1 1 0
0.723145 1 0 1 1 1 0 1
0.731446 1 0 1 1 1 0 0
0.739258 1 0 1 1 0 1 1
0.747559 1 0 1 1 0 1 0
0.756348 1 0 1 1 0 0 1
0.764548 1 0 1 1 0 0 0
0.773438 1 0 1 0 1 1 1
0.781738 1 0 1 0 1 1 0
0.790527 1 0 1 0 1 0 1
0.799805 1 0 1 0 1 0 0
0.808594 1 0 1 0 0 1 1
0.817671 1 0 1 0 0 1 0
0.827148 1 0 1 0 0 0 1
0.836426 1 0 1 0 0 0 0
0.845703 1 0 0 1 1 1 1
0.854980 1 0 0 1 1 1 0
0.864746 1 0 0 1 1 0 1
0.874512 1 0 0 1 1 0 0
0.877441 1 1 1 1 1 1 1
0.884277 1 0 0 1 0 1 1
0.894043 1 0 0 1 0 1 0
0.904297 1 0 0 1 0 0 1
0.914551 1 0 0 1 0 0 0
0.924805 1 0 0 0 1 1 1
0.935059 1 0 0 0 1 1 0
0.945801 1 0 0 0 1 0 1
0.956055 1 0 0 0 1 0 0
0.966797 1 0 0 0 0 1 1
0.978027 1 0 0 0 0 1 0
0.988770 1 0 0 0 0 0 1

 

 

 

GM Axle Ratio RPO’s
RPO Ratio
GM1 2.59:1
GU2 2.73:1
HC2 2.76:1
HE3/G44 3.07:1
GU4 3.08:1
GU5 3.23:1
GU6 3.42:1
GT4 3.73:1
GT5/GT8 4.10:1
HC4 4.56:1

 

 

 

Gear Ratio Tooth Count
Ratio Ring Gear Pinion
2.59 44 17
2.72 49 18
2.73 41 15
2.76 47 17
3.07 43 14
3.08 40 13
3.23 42 13
3.42 41 12
3.55 39 11
3.70 37 10
3.73 41 11
3.90 39 10
4.09 45 11
4.10 41 10
4.11 37 9
4.56 41 9
4.86 34 7
4.88 39 8
5.13 41 8
5.29 37 7
5.38 43 8
5.71 40 7
6.14 43 7
6.17 37 6
7.17 43 6