Certified 120mph speedo swap

[1968 Pontiac]

In TH350 & TH400 trannies one changed a gear inside the tranny to suit the rear end gear ratio; there was no such thing as a different speedo for every different gear set. Thus if you changed your diff gears, you didn't change the speedo, but you changed the tiny gears in the tranny. Even the 9C1 speedo's of the sixties worked in that fashion. Is this no longer true on cable driven speedo's with 700R4 and or metric trannies?

[silicon212]

Originally Posted by 1968 Pontiac

In TH350 & TH400 trannies one changed a gear inside the tranny to suit the rear end gear ratio; there was no such thing as a different speedo for every different gear set. Thus if you changed your diff gears, you didn't change the speedo, but you changed the tiny gears in the tranny. Even the 9C1 speedo's of the sixties worked in that fashion. Is this no longer true on cable driven speedo's with 700R4 and or metric trannies?

No, it's the same as always, at least until the advent of reluctor-driven electronic speedos. Am I losing something in the translation?

The TH700R4/4L60 has the same speedo setup as a 350 or 400 does, exactly the same - a plastic drive gear clip-mounted onto the tailshaft. A TH2004R has its governor mounted in a cavity behind the oil pan - you have to take the pan off to have access to it - the governor drive gear is also the speedometer drive gear - the driven is still the same as the other transmissions.

Most cable-driven speedometer heads have a little lever behind the mechanism that can be used to slightly adjust the calibration of the needle (think of it as fine-tuning for tire sizes), while the 9C1 certified units are sealed and don't have this, but aside from that they are the same.

[silicon212]

Originally Posted by CD Smalley

Just curious, what do those speedos go for on ebay?

I bought a replacement one for mine at a junkyard for $5.

[CD Smalley]

That's good to know. But it wasn't the question. Maybe I should have phrased it like this:

"What are idiots on eBay willing to pay for a 9C1 speedo?".

[PeteA216]

Well, since I went to the 8 different junk yards I could find in my area (roughly within a 40 mile radius) and came up with NOTHING for a certified 120 mph speedo, I'm paying upwards of $40 after shipping for a complete cluster.

[CD Smalley]

I should revize that again and add "Excluding our friend Pete here."

Sorry Pete.

I think you got a great deal. I recall seeing them sell for hundreds of dollars not too many years ago.

[PeteA216]

Hahaha, thanks Smalley. I know, they're not on there all that often either. This one was the first one I've found in about 8 months.

[ernestmayhandsr]

All that information aside, will this 120mph speedo install easily in an '87 with the "upgraded special instrumentation"? I know I'll be sacrificing my voltmeter and temp guage but I'm fine with that as long as the cable will make the speedo read "fairly" acurately (which it sounds like it will from reading all the replies to this) and as long as the gas guage will work off appropriate wires which must already be connected on the back of my current cluster (cause my gas guage works great). I'm guessing the dummy lights that come on this 120mph cluster won't work but I do have a aftermarket cluster for under the ashtray that's got all I need. Maybe the wires for my "upgraded special instrumentation" are completely different and this 120mph speedo won't work at all? thanks