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How to Recalibrate/adjust your speedometer

You might want to measure your actual loaded tire height from road surface to center of hub, I suggest this because its critical in final calibration.

The tire you have 23585/16 equates to 31.73" diameter average however a loaded tire maybe 28" (as example) there various debates about this however; I have always had to adjust for the smaller diameter of the loaded tire (s).

I'm running 295/75/16 advertised as 33.5" loaded they are 31.5" "loaded measurement requires tire to be on proper width wheel and the entire thread contacting the road surface (tarmat).
 
BigBlueChevy said:
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.
0705101210.jpg

0705101211.jpg



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.
0705101212.jpg



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.
0705101213.jpg

0705101214a.jpg



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"
tirediameter.jpg


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":
http://www.tbichips.com/drac/

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.:
http://www.tbichips.com/drac/


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.
Click to expand...
What if numbers don't exactly match?
 
I have a 1997 Chevrolet 1500 5.7l that someone put a 4.11 in place of the 3.42. My question is does it even have the VSSB because I have not found it
 
Robert Champeau said:
So no way to adjust it
Click to expand...
You can. There's only 2 programs I know of that can alter a 96/97 black box, and only 1 of them is still available. Tunercat(pretty sure this one is nla) and ls1edit. 98+ has many more options. I'm not sure if a Tech2 can alter the speedo settings, I've never had a 96/97 to try to adjust. Your other option is dakota digital and a few others make calibration boxes that you can cut the wire going to your speedo, install it inline, and it alters the signal.
 
I heard somewhere that the transmission could be calibrated to clock the revs per mile to give a more correct speedometer reading. I am changing my drive tires soon from 275/80R22.5 to 295/75R22.5. My speedometer is not that accurate now. I don't know what it will be when I change the tires. Is this correctable through Allison? If so, where do I get this done?
 
Hello @deerefan
Welcome to the forum.
If You do not get some replies in this thread then it might be worth starting a new thread and stating the problems.
Whoever has the knowledge about such things will also need to know the make, year, model and how the wehicle is squipped.
 
So this thread is for 6.5 diesels which has a totally different system than your allison will use. You should start a thread in the appropriate area for your vehicle so as to not muddy the water here on this thread
 
Hey guys, I know this is an older thread, but I figured I'd post here since I needed to calibrate my speedo after getting my truck back from installing 3.73? going from 4.10's.

Followed the instructions along with reading a bit on other forums too, did my calculations on the formula and going off the chart. I have 265-75R16 tires which gives a 99.37976377952756 circumference, using the chart on the site linked below gave me a ratio input of 0.7431492810130232 that shows on the chart as the closest setting 0.739258 or 1011011.

before I started I checked my VSSB pins and looked at the chart to see where it was at from factory. set at 0.874512 or 1001100 and after having the gears installed I ran the speedo against my phone's gps and saw my speedo was 5 miles faster than the gps.

switching to the lower setting still showed me the same result 5 mph over the gps. Thinking back when I got the truck how the speedo was off running the stock tires (235-75) I looked at the chart and used tire size 245-75 which gave me a lower number on the chart. After driving up the road checking at 30mph and again at 50mph I was seeing 3mph over on the speedo.

So I did one more run using the chart to set it for tire size 235-75 and 3.73 gears, gave me 0.7872153208675311 which showed on the chart two numbers that were close. 0.781738 ( 1010110 ) and 0.790527 ( 1010101 ).

A test run down the road again showed my speedo very close. About 1mph to only 1/2 mph over my gps. I am thinking there is something wired going on here but at least it’s close now.

And the link to the chart I used

Calibrate TBI Speedometer VSSB DRAC | TBI

Calibrate TBI Speedometer VSSB DRAC
harristuning.com

Here’s the before and after pics of the VSSB in my truck. Of course I replaced the solder joints with some dip switches off Amazon
85EF3B2A-A49B-4B14-B4F4-458E034E3B8B.jpeg88310B6E-CE6F-40F8-8E44-BD0D4ECC7D4D.jpeg
 
I am assuming that by calibrating the speedo, the mileage counter will be similarly accurate for tracking mpg's at least I hope.

I try to track it heading to work and back tomorrow :)
 
For all who might use this thread in the future programming the VSSB I ran across this site showing different info. on the other charts and instructions it shows not to change the two pin #1's on these. this site shows something different, also gives me a different number on the chart to go by which is close to where I set mine. I may try this setting in the next day or so to see if it's closer to my gps.

www.tradecraftspecialties.com

Tradecraft Specialties

Specialized repair and troubleshooting
www.tradecraftspecialties.com www.tradecraftspecialties.com
 
Good morning all, heading to work this morning the first thing I noticed after getting the speedo to read correctly is my overdrive / lockup didn't happen until I was going 50mph rather than 45. I am assuming that the PCM reads the speed differently. I may need to connect the scanner and see what it sees.
 
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