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I did ls swap in 1988 Silverado and changed gear ratio now how do I calibrate my speedo it's the old liquid type speedo
 

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You will have to go to a junk yard and find a truck 88 or 89 with the ratio you installed and get the DRAC out of it. The DRAC is located behind the glove box about 2" square and 3/8 to 1/2" thick.
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Heres a write up a buddy of mine in Houston did. Hope this helps.
yes, there isn't a DRAC or VSSB in the 88 - 91 dashes, the signal converter is on the back of the cluster as Richard showed.

Greg, you're buddies with Richard? Cool, he's still quite active on the GMT400.com forum - small world.
 

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my guess is the rear area of the transmission where the speedo signal sensor has to change the sensor adapter..
now in todays world newer vehicles my 2000 will change the software on the PCM to adjust the correct rear diff ratio..
you just changed the rear diff .. so you have just a 2WD truck !!!!!!!!!
 
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