Speedo, Tach questions.
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Joined: Mar 2008
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Speedo, Tach questions.
I just got my 78 L-82 Anniversary last fall. It hadn't been driven really since 1989. I worked on it all winter, and now I'm drivin it. But the speedo gears were removed long ago. I want to reconnect, but I dont know which gears to get. How many teeth etc. It's a 4-speed muncie. Any suggestions? One more question. My tach seems to only be reading up to 3k or 3.5k. If I'm accelerating hard, once it gets to that rpm the tach starts going backdown? even though the rpm are definately going up. Any advice? This is my first chance to wrench on a vette, and I must say, this forum has been very helpfull!
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
I should be able to get you a number if you know your rear end ratio.On your tach if its running through the filter at the dist remove the wire from the filter and plug it directly into the dist.
Last edited by ...Roger...; May 25, 2008 at 02:34 PM.
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Jack the rear wheels up off the ground with the trans in neutral. Turn the rear wheel one full revolution while counting how many times the driveshaft turns. The number of turns is your rear end ratio. 
(This assumes that you have positraction and both rear wheels turn together.)
Although there may be a 2-letter stamping in the differential carrier, considering the age of the car, the gears may have been replaced with another ratio.
(This assumes that you have positraction and both rear wheels turn together.)Although there may be a 2-letter stamping in the differential carrier, considering the age of the car, the gears may have been replaced with another ratio.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
There are #s on the diff. housing but might be tough to find.If the gears had been changed it wouldnt do much good to find them.When I want to check I jack up the rear and mark the tire and driveshaft.Then rotate the tire one full turn and count how many times the driveshaft turns.On a 3:08 the driveshaft will turn will turn 3 times(3.08)-3:36 will turn slightly more than 3 1/4 times (3.36) 3:55 will turn 3 1/2 times(3.55).
The standard rear end ratio in 78 with a 4 speed was 3:36.If you have a 3:36 the speedo driven gear is 18 teeth,brown in color and a GM# of 3987918.
The standard rear end ratio in 78 with a 4 speed was 3:36.If you have a 3:36 the speedo driven gear is 18 teeth,brown in color and a GM# of 3987918.
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
I just got my 78 L-82 Anniversary last fall. It hadn't been driven really since 1989. I worked on it all winter, and now I'm drivin it. But the speedo gears were removed long ago. I want to reconnect, but I dont know which gears to get. How many teeth etc. It's a 4-speed muncie. Any suggestions? One more question. My tach seems to only be reading up to 3k or 3.5k. If I'm accelerating hard, once it gets to that rpm the tach starts going backdown? even though the rpm are definately going up. Any advice? This is my first chance to wrench on a vette, and I must say, this forum has been very helpfull!
Team Owner
Joined: Jul 2006
Posts: 22,192
Likes: 4,180
From: Kannapolis NC
If it's stock, the tranny will be a Borg-Warner, and my 78 L-82 4 speed came with a 3:70 rear axle. If you find the code on the diff, post it. The only way to tell if it's original is to roll it as above. Count carefully, it's hard to tell the difference in a 3:55 and a 3:70.
