I just installed my 4:10 rear end on my 79 and my speedo is all messed up now.
Tell me what i gotta do to correct it or what i should replace it with.
Help me here.
Gimme suggestions plz.
Thank you in advance.

 

I had that problem when I switched from 2.56's --> 3.42's in my '81 Z28.

Is your car auto or standard? Either way, doesn't really matter, but what the car has is a couple of gears in the side of the tranny where the speedo cable hooks up. One gear is on the output shaft of the tranny, the other goes in from the side. You're going to have to change at least one of, or both those gears.

What was your old ratio?? If it was 3.42 for example, and now you've gone to 4.10, that's a 20% lower ratio, but that will make your speedo read 20% faster than it should ---> ie: 100km/h will now read 120km/h, or 60km/h will read ~72km/h.............does that sound about right?

What you have to do is go to a GM dealership, and talk to the guy in parts. They should be able to pull up a chart that lists the different gear combinations for the speedo drive............I don't know if there is one already determined for 4.10's, but if you know what your transmission currently has, you can "customize" a combination that will work. You might need a drive gear with 1 or 2 less teeth, and maybe a driven gear with 1 or 2 more teeth (to slow down the overall read-out to the speedo by ~20%).

I hope that's clear enough for ya. If you've got any more questions, feel free to e-mail me, or PM me.