which speedometer driven gear do i need
Thread Starter
Racer
Joined: Mar 2011
Posts: 252
Likes: 3
which speedometer driven gear do i need
How do I know which speedometer driven gear that I need? Before I started my restore, the speedometer was slow by about 10 mph at 60 mph. Car is 79 L82 with a TH350. Also, fluid was leaking from that area.
Any help would be appreciated.
gary
Any help would be appreciated.
gary
Race Director
Joined: Oct 2000
Posts: 12,680
Likes: 3,134
From: Shamong, NJ
There are plenty of speedometer gear calculators on the net. Try www.gtsparkplugs.com/SpeedoGearCalc.html. You'll need to know what rear gear ratio you have, and the size (diameter) of your rear tires, to get an accurate answer.
Thread Starter
Racer
Joined: Mar 2011
Posts: 252
Likes: 3
There are plenty of speedometer gear calculators on the net. Try www.gtsparkplugs.com/SpeedoGearCalc.html. You'll need to know what rear gear ratio you have, and the size (diameter) of your rear tires, to get an accurate answer.
Burning Brakes
Joined: Mar 2010
Posts: 803
Likes: 4
From: Christchurch, New Zealand
I'm in the same predicament. Only mine's reading 10mph too fast. Use Search on the forum, and you'll be sweet! I found I need a 35 tooth gear. You'll need to know your total wheel diameter, rear end ratio, and obviously, manual or auto.
Burning Brakes
Joined: May 2004
Posts: 1,201
Likes: 2
From: Frederick Md
Pull gear out of the side of the transmission...(easy to do in a minute or two) and count the teeth....
since you are reading 10 mph LOW then you need a fear with TWO less teeth....
if you were reading Fast you would need a gear with one less tooth for every 5 mpg...
They only cost about 10 bucks each...I purchased mine from Dr. Rebuild..
Bob G.
since you are reading 10 mph LOW then you need a fear with TWO less teeth....
if you were reading Fast you would need a gear with one less tooth for every 5 mpg...
They only cost about 10 bucks each...I purchased mine from Dr. Rebuild..
Bob G.
Safety Car
Joined: Sep 2007
Posts: 4,458
Likes: 331
From: Indy Indiana
yes you will need to lower the tooth count by 1 or 2 teeth, you get about 5-7 mph difference per tooth.. buy them both, they are cheap ( and pay only 1 shipping cost) and just try them.
you have tranny slip, clutch slip, tire wear, speedometer calibration itself... all will cause your speedo to be off.
you will need a new o-ring at that conection also to stop the leak.. you will see it when you pull the old one out.
you have tranny slip, clutch slip, tire wear, speedometer calibration itself... all will cause your speedo to be off.
you will need a new o-ring at that conection also to stop the leak.. you will see it when you pull the old one out.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
on a 1979 the driven gears used were:
3:36 ratio used 18 tooth
3:55 ratio used 22 tooth
3:70 ratio used 20 tooth.
Based on the tires being the original size. If I had to guess I'd say you have a 3.55 rear end ratio.
Here is the GM chart:
On the leaking issue. Replace the o-ring on the fitting and the seal on the gear. But also inspect the shift shaft seal just forward. It is common for this shaft seal to leak and when it does it blows back on the speedometer gear fitting.
If it is leaking you'll need a special tool to remove the old seal. Never try to pry out the shifter shaft seal it will score the shaft and then you'll really have your hands full. We see this all the time... When the shifter shaft is scratched up it will eat the seals as fast as you can put them in.
Tool for removal of shifter shaft on AT cars:
The part number on this tool was t-1001. You can find them cheap in an aftermarket set.
Willcox
3:36 ratio used 18 tooth
3:55 ratio used 22 tooth
3:70 ratio used 20 tooth.
Based on the tires being the original size. If I had to guess I'd say you have a 3.55 rear end ratio.
Here is the GM chart:
On the leaking issue. Replace the o-ring on the fitting and the seal on the gear. But also inspect the shift shaft seal just forward. It is common for this shaft seal to leak and when it does it blows back on the speedometer gear fitting.
If it is leaking you'll need a special tool to remove the old seal. Never try to pry out the shifter shaft seal it will score the shaft and then you'll really have your hands full. We see this all the time... When the shifter shaft is scratched up it will eat the seals as fast as you can put them in.
Tool for removal of shifter shaft on AT cars:
The part number on this tool was t-1001. You can find them cheap in an aftermarket set. Willcox
Last edited by Willcox Corvette; Nov 15, 2011 at 01:00 PM.
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Instructor
Joined: Apr 2011
Posts: 161
Likes: 17
From: NW CT
Sorry to bring up a dead thread.. But what book is that chart from ?
I don't see it in my factory service manual.
I have a 1977e. It has a 1977 TH350 in it but out of another type of car. My speedo is high 10mph. 60mph reads 70mph.
I want to find out what stock color gears are and try and and figger out what my rear end ratio is. Its a l48 auto.
I don't see it in my factory service manual.
I have a 1977e. It has a 1977 TH350 in it but out of another type of car. My speedo is high 10mph. 60mph reads 70mph.
I want to find out what stock color gears are and try and and figger out what my rear end ratio is. Its a l48 auto.
Race Director
Joined: Oct 2000
Posts: 12,680
Likes: 3,134
From: Shamong, NJ
77's with the L-48 and an automatic, came with a 3:08 rear. If you'd had an L-82 with an automatic, it would have come with a 3:55. There were no optional ratios offered with the automatics.
The chart that Willcox posted, came out of the Corvette Parts Book. I don't have any of my Parts books near by at the moment, but the speedo gears for the 77, I think are the same as those for the 78, in the chart above.
The chart that Willcox posted, came out of the Corvette Parts Book. I don't have any of my Parts books near by at the moment, but the speedo gears for the 77, I think are the same as those for the 78, in the chart above.
Instructor
Joined: Apr 2011
Posts: 161
Likes: 17
From: NW CT
77's with the L-48 and an automatic, came with a 3:08 rear. If you'd had an L-82 with an automatic, it would have come with a 3:55. There were no optional ratios offered with the automatics.
The chart that Willcox posted, came out of the Corvette Parts Book. I don't have any of my Parts books near by at the moment, but the speedo gears for the 77, I think are the same as those for the 78, in the chart above.
The chart that Willcox posted, came out of the Corvette Parts Book. I don't have any of my Parts books near by at the moment, but the speedo gears for the 77, I think are the same as those for the 78, in the chart above.
This info should make sorting out this issue alot simpler.
