What do I need to change in the ECM to let my speedo work correctly with my newly installed 3.42? I used to have a 2.73. Thanks.

 

well, one way is to change the gears on your VSS for the gears out of a 3.42 car.

otherwise, (i'm assuming you are running '730 / $8D with an electronic speedo?), you need to change the road speed constant and the instrument panel divisor ratio. the IP ratio is a fairly coarse adjustment, and you may find that it doesn't get you close enough.

there is some posts in the archives on the topic of the divisor ratio.

personally, i'd order the VSS gears out of a t5 3.42 application, i believe they would work. If you're lucky, the drive gear on the tailshaft won't need to be changed.

there are also little boxes you can buy that translate/scale the VSS signal up or down as needed. GM has put them in some of their cars, dakota digital makes one, and someone else makes one too. don't have it in memory off hand, but should be in the archives too.

so, there's a couple of different ways you can go.

 

There was some research done on the divisor but I have no experience with it. Post your results if you play with them so more info on the subject is available.

An old post about adjusting the bits at the instrument panel divisor ($8014)
https://www.thirdgen.org/techbb2/sho...t=reads+96+mph

 

My experience with the divisor worked exactly as that thread describes.

I was able to utilize it when I went from 700r4 to T56. One of the values happened to be perfect for me and the special T56 VSS.

 

Someone also mentioned to me that I could buy the correct gears to go on my VSS in the tranny. Would GM sell those? Where would I get the correct gears?