Speedo Gear question
Thread Starter
Melting Slicks
Joined: Sep 2002
Posts: 2,390
Likes: 2
From: New Castle DE
Speedo Gear question
Ok my speedo is off 10 mph...when i am doing 60 it reads 50. I have confirmed this during cruises with friends and it is always off 10 compared to like 6 different cars. My question is how do i know what gear to get to replace it and do i replace the one in the tranny or is there one in the dash? I think i have a 3.08 rear but am trying to confirm that as well. Where is the code on the housing to check for ratio. I am alos going to just jack up the car and rotate the tires to make sure.
Race Director
Joined: Apr 1999
Posts: 10,866
Likes: 717
From: Beverly Hills/Pine Ridge Florida
Pull the gear in the trans. It's color coded. You can count the number of teeth on the gear.
You need one to spin faster to make your speedo read "higher", so you want a gear with FEWER teeth.
You'll have to "do the math" to find the right # of teeth.
Chuck
You need one to spin faster to make your speedo read "higher", so you want a gear with FEWER teeth.
You'll have to "do the math" to find the right # of teeth.
Chuck
Race Director
Joined: Apr 1999
Posts: 10,866
Likes: 717
From: Beverly Hills/Pine Ridge Florida
Hmmm. Well, 50 divided by 60 = .833.
So, you need a 17% change (inverse of 83%)
So, if you have 20 teeth on the current gear, you need 17% fewer teeth.
20 X .17 = 3.4. So, get a gear with 17 teeth (in this example only) Chuck
So, you need a 17% change (inverse of 83%)
So, if you have 20 teeth on the current gear, you need 17% fewer teeth.
20 X .17 = 3.4. So, get a gear with 17 teeth (in this example only) Chuck
Race Director
Joined: Apr 1999
Posts: 10,866
Likes: 717
From: Beverly Hills/Pine Ridge Florida
Another "way" to do it is to take 83% of 20 (in the same example)
.83 x 20 = 16.6 or a 17 tooth gear.
Some sites say "approximately" 5 MPH per tooth, but that isn't so, especially with big changes.
You don't say if you have a THM or a manual. THM gears have LOTS of teeth 35-40+. 4 speed gears have less, 18-26 ish as I recall.
Be careful pulling the gear out of a 4 speed. It'll drop back into the tail-stock. You'll hate yourself if that happens, as you'll feel obligated to pull the gear out, and often that's very difficult.
The gears are plastic, BTW.
And, to add to your anxiety,
There are 2 different "drive" gears within the trans. You have to use a driven gear compatible with that drive gear.
I'd have to look up the specs. Chuck
.83 x 20 = 16.6 or a 17 tooth gear.
Some sites say "approximately" 5 MPH per tooth, but that isn't so, especially with big changes.
You don't say if you have a THM or a manual. THM gears have LOTS of teeth 35-40+. 4 speed gears have less, 18-26 ish as I recall.
Be careful pulling the gear out of a 4 speed. It'll drop back into the tail-stock. You'll hate yourself if that happens, as you'll feel obligated to pull the gear out, and often that's very difficult.
The gears are plastic, BTW.
And, to add to your anxiety,
There are 2 different "drive" gears within the trans. You have to use a driven gear compatible with that drive gear. I'd have to look up the specs. Chuck
Thread Starter
Melting Slicks
Joined: Sep 2002
Posts: 2,390
Likes: 2
From: New Castle DE
great now your scaring me 
lol. It is a 4 speed and if anything can happen to make what i am doing more difficult it always does. So i am sure i would drop that puppy into the tailshaft.
lol. It is a 4 speed and if anything can happen to make what i am doing more difficult it always does. So i am sure i would drop that puppy into the tailshaft.
Race Director
Joined: Mar 2001
Posts: 11,322
Likes: 1
From: Living in the Hartland
believe 1 tooth is about 4 mph so you will need one with either 2 or 3 teeth less, see if you can get the reading a little closer so you tell how far you are off at 60 mph
btw the gear goes into the output fitting so you should not have a problem with dropping it into the tail shaft hopefully
btw the gear goes into the output fitting so you should not have a problem with dropping it into the tail shaft hopefully
Last edited by Fevre; Aug 23, 2006 at 02:34 PM.
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Heel & Toe
Joined: Aug 2006
Posts: 24
Likes: 0
From: Mountain Home AFB ID
I have the same problem...
A chevy dealer told me a tooth was missing in a gear off the trans end.
I give the dealer a big BS Flag!
Glad I joined this forum!
When I get my garage this fall its *******s and elbows to start gettin my baby in shape!
A chevy dealer told me a tooth was missing in a gear off the trans end.
I give the dealer a big BS Flag!
Glad I joined this forum!
When I get my garage this fall its *******s and elbows to start gettin my baby in shape!
Heel & Toe
Joined: Aug 2005
Posts: 21
Likes: 0
From: Acworth Georgia
Go to dealer and suggest a car that came with the size tires you have. The entry in the chart under that tire size for your differential ratio will list the correct part numbers for the drive and driven gear. For example, I had the same problem with a 1970 Chevelle. I had installed a 3.08 differential gear and 25560R15 tires. Since I knew that a 1980 Corvette came with tires of that size, I 'helped' the parts guy by suggesting that he look under 1980 Corvette. There was an entry under 25560R15 for a 3.07 rear end. He ordered both speedometer gears and the Chevelle speedometer reads properly now.
