I have a rebuilt automatic 700r4 in my 92 RS with a weird speed reading issue. The speedometer in first gear is reading about 6 miles higher, third gear is 11 miles under, and second gear is some weird gray area in between. I traced the car’s VIN to find it was originally a manual. The person I bought it from said one of the previous owners had replaced the trans with a rebuilt one (this one being automatic) from “another Camaro” with no other details other than the donor car being painted blue with white doors, which the seller pointed out was California’s patrol colors. Not sure if I believe him, or that it’s relevent, but I’m hoping that detail could maybe help. I’ve read from another forum that the trans should have the serial number casted/riveted to the side, so after looking around, I’ve found the following:
2 (riveted to the side, the rest is casted)
10031031
MD8
8676358
8673553-2
After a little bit of Googling, I’ve found that the last set of numbers resembles a 94-96 model the closest but i have no idea how accurate that is. I know that the driven gear is wrong since it is brown instead of the stock gray, and I haven’t checked the rear end yet. My tires are 225/60/16, but I don’t see how any of that would affect the speedometer that drastically on each gear? I have no idea where to even begin to fix this, or what could possibly be the problem. I was thinking maybe the speedo chip itself is still programmed for the manual transmission, and converting the trans reading incorrectly. I’ve also watched it zero out for a second while driving, so I’ve been planning on replacing it anyway. I’m praying that it’s not the transmission itself going bad.
I’m new to just about everything involving cars and tired of searching with no results, so I would greatly appreciate at least a little bit of guidance, but no one I know has a clue what’s going on.

The third gens with electric speedo still had speedo gears driving a VSS in the tailhousing.
you can pop it out pretty easily and count teeth of the driven gear. Also, with a magnet, a. Sharpie, and patience, you can count the drive gear.You will also want to change the lube in the differential, and count the ring and pinion teeth so you know what it has. The tires you have are taller. That, plus the non-original trans. mean you could have a couple things affecting accuracy.

https://www.thirdgen.org/forums/tran...eedometer.html is a good read

The gears can be changed out to correct it, but a pen adjustment box could go between the trans. VSS and the ECM, like a Dakota Digital SGI5E. 94 production was 100% 4L60-E.

Apologies for the late response, I was working with my fuel priming issue first which has been fixed. I’ve got the SGI-5E and plugged it between the ECM and VSS and the speed was pretty accurate at L-L (maybe one or two mph off at highway speeds) but the car shut off at 60mph. Gas was full, battery fine, everything plugged in correctly. Started back up again, got to 60, then shut off again. I haven’t had a governor shut a car off before, but I’m pretty sure that’s what happened. I kept the interface unplugged and drove at the same speed and the speedometer (which normally shows anywhere from 47-50 at 60mph) kind of flickered between 50, 40ish and 80mph. Since then, even with the interface unplugged and all wires matching and in tact, it flickers between speeds every now and then. Some times more often than others. And on few occasions, it doesn’t read anything—maybe 40mph for a split second at highway speed. If the governor kicked in at 60, then it’s reading when it flickered into 80mph, right?
My fuel priming issue was that two wires had been accidentally cut and melted together from the engine heat, one going to the cut off switch and the other being constant hot. I’m wondering if something similar happened with the speedo wire, but I’m not sure how the interior could have gotten so hot?
I’m in the process of cutting out the speedometer wire between the gauge and ECM altogether and replacing it with a new wire, so maybe if there is a conjoinment, it’ll avoid it entirely.
Any ideas?

I forgot to mention that when I unplugged the interface after the governor kicked in twice, we had taken out the fuse that we installed in the wire that connected the two halves of the vss input. That means that the ECM should not be reading any speed at all, but was reading... something. The speedo maxed at about 15mph in multiple flashes of signal while driving at 60mph during the time.

Update: Bypassing the wire didn't change anything, so I'm thinking that the VSS itself is the issue. I'll look into a new one.