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.
The 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.
There has been an ebay seller making the vssb with the dip switches installed. With that you dont have any risk to original and easily just use calculator below.
| 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 |