Hi all,

I’m having speedo cable or gauge issues (maybe both) but looking into that has raised another question.

My car is a ‘66 427/390 4spd 3.70 rear end.

At the tranny, I took the speedo cable and the round fitting out and the driven gear is brown and 18 teeth.

I read where for a 3.70 rear end the gear should be green and 22 teeth.

Since I got it, the speedo has been off a bit but I never wrote down exactly how far off. I was thinking 5-10mph.

If I went to a brown gear it would change the speedo like 20mph.

Not sure why the gear is brown or what to do. The parts books say green 22 teeth is for a 3.70.

Seems like a big change.

 

I just went through this on a recent Muncie fix. Before the pumpkin was changed out for a 3:08 the rear gear held 4:11’s.
At the time running the 4:11, I wasn’t aware of the different sized drive gear on the output shaft and how this was related to final drive ratio.

It appears my car originally came with a 3:36 final drive ratio. The only evidence was the blue 20 tooth driven gear that came out of the bullet when it was swapped for a ‘correct’ 25 tooth orange colored gear.

It’s been quite some years but I think I remember noticing the difference in size between the two driven gears but figured it was because of more teeth = larger gear. A duh moment that should’ve been a
clue!

Anyway, in goes the orange gear. The mph read out was pretty close........3000 rpm @ around 60 mph with the 4:11. The car was driven that way for years. Why the orange gear didn’t break or get chewed up I don’t know. All I know is it worked lol.

With the recent Muncie fix a correct for 3:08 brown driven gear was purchased. On a recent drive, cross checking with the GPS shows speedo reading 5 mph high. A step up to a 19 tooth driven gear would bring
the mph down (5 mph differential between tooth count) to fairly correct, but the tooth difference between brown and blue gears is 2 teeth (18 vs 20).

Might try the blue gear again......just to see difference.

John

 

There is a 19 tooth, GM 3987919, natural (white) gear.

 

Why don't you just "do the math" and calculate what you need?

Your phone should have a GPS. Drive down the road at 60 MPH on your GPS...and write down what your speedo shows.

Let's say the speedo shows 60 MPH when you're actually going 50 MPH on the GPS. You have to slow the speedo down. You need a gear with more teeth.

60/50 = 6/5 = 1.2

Your current gear has 18 teeth. 18 x 1.2 = 21.6... Round up to 22 teeth.

 

Interesting that you know so much about teeth......:-)

 

22 is the one you need. I just changed one out on a 67 with a 3:70.

 

I should have checked. I wasn’t aware of the 19 tooth gear. ..Thanks

John

 

A 1 tooth change is about 5 percent, whixh is 5 mph at 100mph and 3mph at 60mph.

 

Thanks guys. I know I need the brown 22 tooth gear (and ordered it) with a 3.70 but depending upon what you read that could result in a 20mph change.

I don’t think my speedo was off that much but now the speedo has failed, that’s another deal.

I was kinda wondering if the resto 10yrs ago could have put an incorrect tranny drive gear in or some other deal with the tranny to effect the speed and have made them choose the wrong driven gear.

I appreciate the responses.
Dennis

 

You just posted that you ordered a brown 22 tooth gear. I think you meant to say green, because the brown gear is 18 teeth. The LI corvettes catalog has all the info on this stuff.. You need a 22 tooth (green) gear for a 3.70, but if the drive gear is incorrect for some reason (maybe the car was born with 3.36 or 3,08), there is a silver 22 tooth gear for use with the other drive gear.
Ray