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How to Recalibrate/adjust your speedometer

Discussion in 'Do It Yourself Articles - How To's & Product Revie' started by BigBlueChevy, Jul 5, 2010.

  1. BigBlueChevy

    BigBlueChevy Compression Ignition Addict

    Feb 22, 2009
    This thread will provide detailed instructions for those with a 1992-1998 Gm pickup truck, with either 2wd or 4wd who wish to recalibrate there VSSB(vehicle speed sensor Buffer). This should be done when gearing ratio or tire sizes are changed to keep both your speedometer and Odometer correct.


    A few early precautions.
    • Whenever you work with electronic equipment(specifically the PCB) you are dealing with extremely sensitive electronics. Static electricity is FATAL to electronics. Do not touch any of the solder joints with your hands or other tools not ESD safe. Static electricity can destroy boards and you won't feel a damn thing.
    • Avoid excess solder, bridging a solder connection between two solder holes can cause damage and "unwanted" results. You are working with parts normally assembled by either machinery or under microscope. Always double check your work.
    • Adding the DIP switches to replace the jumpers is not really required. Although, for those who switch tires sizes often or own 2 different sets of tires(say drag and street, or summer and Snow) it is a very worthwhile investment. It will not be covered in this writeup however.


    Start by locating and removing your VSSB unit. It will be located near the PCM, right underneath it(results may vary) The PCM is located behind the glovebox as show in the pictures below. Disconnect the wiring harness and remove the VSSB. Its only held in by 3 clips so pulling on the case a few times will get it loose.

    Once you have the VSSB in hand, bring it to a safe location and remove the PCB from the protective white case. there are two clips on either side, push them in and the case backing will flip right up.

    Remove the PCB by pushing up and away on the black connector protruding from the plastic. When handling the board, try to only contact the plastic connector. This will help avoid accidental static discharge.

    Now the fun part. The pins/jumpers that we will be adjustming are located on the bottom of the PCB. But first, use the following charts, and websites to gather all of the technical data you need before melting any solder.

    1. You will need to determine your gearing ratio in your differential(s). This can be done by checking the RPO codes located in your glovebox for one of the following:
    • GU6. You have a 3.42 to 1 gearing ratio
    • GT4. You have a 3.73 to 1 gearing ratio
    • GT5/GT8. You have a 4.10 to 1 gearing ratio.
    • HC4. You have the big daddy 4.56 to 1 gearing ratio.
    • Next, you will need to determine your tires rolling circumference. Not as hard as you think, ahhh the power of the internet. Use the following website to input your tire info and get the rolling circumference you need:http://www.5speedtransmissions.com/calculators.html

    Using the above website. Enter your tire information. As you can see in the example below, my tires are a 315/70R17. Input those into there respective boxes and click calculate. The information you want is the "tire Circumference(in=)". Which I've circled below in the example. Another example:
    -A stock 245/75R16 tire will have a rolling circumference of 95.8"
    -A slightly larger 265/75R16 will have a rolling circumference of 99.3"

    Now, you will need to take all of this new found info and add it all up. This can be done with the formula on the following page labeled "input ratio":

    Your going to want to use a scientific calculator to perform this equation(for the parenthesis). Most computers these days have calculators that can be run as a scientific type.

    The forumla will look something like this on a calculator:
    (63360X4.10X40) "DIVIDED BY" (108.1X128000)
    Your formula will differ than mine depending on your gearing ratio and tires rolling circumference, but if done properly you will have a long number to compare to the chart on this page labeled Jumper settings It is the third chart from the bottom of the page.:

    My input Ratio came out as 0.750971323. Compared to the chart on the page linked above, I will have to arrange my jumper's as followed:
    1 2 3 4 5 6 7
    I 0 I I 0 0 I

    I= jumper needed.
    0= left open.
  2. BigBlueChevy

    BigBlueChevy Compression Ignition Addict

    Feb 22, 2009
    Once you knew where everything has to be soldered, modified and or removed, go to work. Be sure to take your time and be careful with the components. Note that the bottoms on the jumper wires are bent inwards to avoid them from being removed accidentally, you will need to bend them straight in order to remove the jumpers from the bore. The link below shows you a picture of how the VSSB has "2 sides too it". The left 7 pins will be inverse to those in its right. Meaning, if Speedo jumper 3 in the right side is open, then speedo jumper 3 on the left side will need to have a jumper or "closed"

    VSSB after modification: I did a terrible job with the jumpers here, having to make them out of paper clips(its ok to do this) Hopefully yours looks alot cleaner than mine.

    And thats all there is too it! Your VSSB has now been modified to compensate for whatever changes you have done to your gears and or Tires!

    Take everything for a test drive in order to ensure the unit is functioning properly, and either have a vehicle pace you at X speed or use a GPS with speed tracking capability. If you need to, make any modifications you feel are necessary. Some people have had varying results by changing pin placement to the "next row down" on the charts listed above.
  3. great white

    great white Well-Known Member

    I use a set of DIP switches in place of the jumpers.

    Makes it instantly adjustable with the flick of a switch.

  4. BigBlueChevy

    BigBlueChevy Compression Ignition Addict

    Feb 22, 2009
    I felt lazy:D Used to work for an electronics company and they spoiled me with all kinds of great soldering equipment. Wish I had the solder vacuum they use there to remove solder from the holes, or hell at least solder wick. Didn't feel like fighting with the solder trying to slip the DIP switches in so I just said screw it. MY basement isn't exactly an ESD safe environment, so I didn't want to push it.
  5. I have the earlier style DRAC (7 jumper) with dip switch so it's adjustable.

    I found that it's much easier to use a GPS and adjust the dip switches while moving to adjust the speedo. This worked great in my truck, however might cause problems with electronic transmissions. Anyway, IIRC you start left to right and it goes from higher adjustments to smaller ones.
  6. buddy

    buddy New Member

    I wanted to also recommend the dip switches. The calculations bring it close but tires are always a little different than advertised and after loading and rolling on the highway. I have had to go two values away after the calculations and I plan on changing tire size next time.
  7. Burning oil

    Burning oil LeroyDiesel.com Vendor

    Jan 3, 2009
    dieselpage had a write up on this many years ago. I had dip sw on one truck. Next truck I went to pull a part junk yard and grabed a bunch on VSSB's and keeped tring different ones until I found one that worked. I was supprized it worked, but it did and its right on too.
  8. Shorty

    Shorty New Member

    Nov 2, 2011

    My 98 k2500 HD does not have this ECM set. Any other suggestions on this topic for a 5.7L vortec truck?


    May 9, 2008
    5.7L VORTEC requires PCM reprogramming to correct the speedo. The VSSB applies to 6.5 trucks, and 95- gassers.
  10. carl6405

    carl6405 New Member

    Jul 12, 2012
    Chesterfield, Va
    I swapped my ring and pinion. Went from a 3.42 to 4.11. I went through this mod and installed the 2 6-pin sets of dip switches. I have the speed dead on now and used a gps to verify.
    However, this mod has created an issue with my pcm. Now, when I go over 75mpg or somewhere between 70 - 75, I get a DTC-71 (cruise control coded) doesnt matter if i am using cruise control or not.
    Anyone else experience this?

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