Speedo gear chart vs. math...
I have had these gears in and out many times and I am sure they are 3.55s. I have verified the clocking of speedo gear in trany.
Here are the readouts for rpm vs speedo reading.
2000 rpm = 25 mph
3000 rpm = 40 mph
4000 rpm = 50 mph
The speedo action is smooth and steady..
Stable but not linear
From 2000 to 3000 is 1000 rpm and 15 mph.
From 2000 to 4000 is also 1000 rpm, but now only 10 mph. 33% less..... something does not compute
Now here's the stumper ;the speedo is supposed to be linear.
At 3000rpm you should be going 63 mph. This is 21mph per 1000 rpm. So at 1000 it should read 21, at 2000 it should read 42 and at 3000 it should read 63mph.
Now looking at your install, at 1000 rpm it would theoretically be reading 12.5mph so 12.5 mph per 1000. Therefore at 3000 it should be reading 37.5 and it's pretty close.
But working the 4000/50 reading , backwards, that's also 12.5mph per 1000; so 3000 should be 37.5 and 2000 should be 25
So my guess is you just rounded the 3000 figure up to 40 from that calculated 37.5. So if that is true, then your speedo head looks to be working correctly , allbeit too slowly.
It needs a boost from 12.5 mph/1000rpm to 21mph per 1000 rpm. This is a difference of 60%. Therefore if you have a 34 tooth in there, you would need a .60x34=20.4, gee where have we seen that number before? Lemmee guess, was it in post #10,lol; why yes I think it was.
Which is still preposterous.
Preposterous, because I've never seen anything like that in an A-body of your vintage.
But the math doesn't lie so get yourself a 20/21 and see what happens........ I guess.
You know what; I did see that once, when a guy installed a metric convertor on the cable. Guess what the ratio was Hyup .62 or 62% But it was the other way 62mph is about 100 KPH so that's .62 in reverse or 1/.62=plus 61%
Ima thinking you should check every inch of your cable, looking for that multiplier, starting right at the trans. It's a little rectangular cube about 2.25 x 1.25x 1.125 IIRC, with the cable coming out of one side. and the other may be directly attached at the trans end or somewhere up the cable.