Speedo after gear a change

[adfac]

Recently, I had the rear end of my 1986 automatic converted from a 2.73 to a 3.54. Everything was successful. So now my speedometer doesn't read correctly. I figure it's about 20 mph off. Can anyone tell me what "driven" gear goes on a 1986 with a 3.54 and does anyone know where I can even find one? And is that all you have to do or is there something else? I have no idea where to go from here. I bought a Dakota Digital SGI-5 universal signal interface from Summit figuring that might be the easiest way to go but it's not specifically for a corvette and doesn't tell you what wires go where and where you can find them on an 86 harness ... so I'm abandoning that idea. It's over my head. I'd really like to do this right so if anyone could offer any help with what to do next to get my speedometer back to reading correctly, I would really appreciate it. Thanks.

[boomdriver]

Just get the signal generator off of a car that had the gear that you now have installed...Lots and lots of GM cars had the same trans...just be sure about tire size etc. The part is out there.

fleabay is a good place to look. Search there and find some sellers (salvagers) that you can contact.
Many trans shops can get the correct gear to fit your trans...its all documented and very easy to look up. There used to be speedo shops in every major city but those have fallen by the wayside with solid state electronics being gthe norm instead of mechanical speedo drive units......now you have to find an auto electric shop, or a trans shop that does speedo service. Absolute worst case scenario would be having to change the tail housing that contains the drive gear & generator,. and thats cake...

If you find the part searching fleabay the swap is easy...getting under the car is the hardest part.

[John Robinson]

maybe this will help you

NovaResource - Recalibrating GM Speedometers

[boomdriver]

I've seen a chart somewhere that had all the "need to know" stuff in order to do a gear swap and do it with the correct part...it was a chart that listed the rears, tire/wheel size/diam and a cpl other things. Had the replacement gear size listed for whatever combo of wheel/trans/rear you could possibly come up with...If I can find that in my vast warehouse of information that I needed one time and stored away....I will post it for reference...
but don;t hold yer breath. That "warehouse of info" is large and somewhat ..uhhhhhhh ...unorganized ??

[adfac]

Factory 3.54 had
Grey 15 tooth drive gear GM # 8642620
Green 42 tooth driven GM # 25513049


Thanks OZ. I'll get on it.

[adfac]

maybe this will help you

NovaResource - Recalibrating GM Speedometers

Thanks John.