Speedometer Calibration Help!!!
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Joined: Aug 2004
Posts: 65
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From: Lakeland Florida
Speedometer Calibration Help!!!
I have a 1973 wih an automatic. the rear gear was changed to around a 4.00. The speedometer was off approximately 25% (showing 80 and doing 60). I took the transmission gear out (behind the cable) and it had a 35 tooth gear. I replaced it with a 45 tooth gear. I was told each tooth changed it by 2.5%. This made very little difference in it. I can't figure out why it would not have changed it one way or another. It did help some however only a couple mph. What went wrong or where did I miss something? Thanks for your help!!!!!
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
I have a 1973 wih an automatic. the rear gear was changed to around a 4.00. The speedometer was off approximately 25% (showing 80 and doing 60). I took the transmission gear out (behind the cable) and it had a 35 tooth gear. I replaced it with a 45 tooth gear. I was told each tooth changed it by 2.5%. This made very little difference in it. I can't figure out why it would not have changed it one way or another. It did help some however only a couple mph. What went wrong or where did I miss something? Thanks for your help!!!!!
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
something is remiss....I THOUGHT from the speed/ratio/tires calculations on the internet, with a proven tach...that I was going 150 mph....RONG.....the Garmin has proved the speedo is now correct up till 70 mph.....dead nutz on...NOW at 80, it's only 78, at 100 it's only 94 so I can see the increase in non linearity of response to the needle movement....
knowing how/why they work....I find that impossible to beleive somehow....
so by the above two check points...it's a joke, these speedometers are not worth CRAP....junk device from word go...and it's supposedly in good shape too.....
Little EVA....
I guess I have to build my own speedo, and no it's not a bike racing outfit either....
.....Tach too....this stockish analogue crap is just that....crap....
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
Burning Brakes
Joined: Aug 2004
Posts: 971
Likes: 7
From: roanoke va
I changed the gear in on the tail shaft of the transmission. I found a breakdown in an old chevy parts book that listed the correct driven gear and drive gear for a 69 of 70 after that they went with another gear in the trans. (plastic) and the gear I installed was a metal gear. I do not have the part number for the gear but if you find the book that shows the 58 to 72 parts breakdown there is a chart in there showing the gears you need.
I have not had the chance to try it out, but I am pretty sure it should be close.
I just looked at wilcox's web site. They have the breakdown I was talking about, but they only have 4 speeds listed.
You may send them a message or call them, I am sure the pages they have scaned came from the parts book and they just need to look it up for the tire size and gear ratio.
I have not had the chance to try it out, but I am pretty sure it should be close.
I just looked at wilcox's web site. They have the breakdown I was talking about, but they only have 4 speeds listed.
You may send them a message or call them, I am sure the pages they have scaned came from the parts book and they just need to look it up for the tire size and gear ratio.
Last edited by wills670; Apr 10, 2009 at 10:32 PM. Reason: add info.
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Burning Brakes
Joined: Aug 2004
Posts: 971
Likes: 7
From: roanoke va
This is the book I am talking about.
http://cgi.ebay.com/ebaymotors/Corve...1%7C240%3A1318
Find somene with the book and the numbers are in there.
Here is another page and I beleive this is the gears you need to try and then go from there.
http://www.corvetteforum.net/c3/vette73/S_SPEED.htm
I looked up the part number for the gear in the trans. #1362293 and it is a steel gear just like I installed in my 72.
http://cgi.ebay.com/ebaymotors/Corve...1%7C240%3A1318
Find somene with the book and the numbers are in there.
Here is another page and I beleive this is the gears you need to try and then go from there.
http://www.corvetteforum.net/c3/vette73/S_SPEED.htm
I looked up the part number for the gear in the trans. #1362293 and it is a steel gear just like I installed in my 72.
Last edited by wills670; Apr 10, 2009 at 10:48 PM. Reason: add info.
Instructor
Joined: Jan 2009
Posts: 100
Likes: 0
From: Dimondale Mich
In my 82 I changed from 2.87 to a 3.73. I would have to run 94 mph on the speedo in order to cruise @ 70. The speedo only reads 85, I'd be pegging it. The original speedo gears set was 19t drive/42t driven (2.87:1 diff)
I used this website for calculation (www.tciauto.com) plug in the info and it should tell the closest gear set. You'll have to know what your drive gear is first. You can pull the driven gears assembly out and count the teeth or look at the color, there is chart on the web that will tell what tooth it is by color. Mine worked out to be 15t drive/42t driven. So I had to change the drive gear on the tranny tail shaft.
I used this website for calculation (www.tciauto.com) plug in the info and it should tell the closest gear set. You'll have to know what your drive gear is first. You can pull the driven gears assembly out and count the teeth or look at the color, there is chart on the web that will tell what tooth it is by color. Mine worked out to be 15t drive/42t driven. So I had to change the drive gear on the tranny tail shaft.
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
I'm having some trouble with my speedo calibration too. My 80 came with a new NOS 140 MPH GM speedo in it. It only had 2,000 miles on it and I even got the original GM box it came in. The car's L82 motor had been blown up and the car sat for over 11 years.
Before I put the new motor and tranny in it I used a several different calculators on the Internet to figure out my drive and driven gears. I've got 3.07 gears and 27" tall tires, the tranny is a 700R4. I used 17 drive and 39 driven gears. This should be right on according to every calculator I used.
The speedometer is slow by a little over 35%. I checked the odometer against mile markers and it is off by 35% as well. ??????
Before I put the new motor and tranny in it I used a several different calculators on the Internet to figure out my drive and driven gears. I've got 3.07 gears and 27" tall tires, the tranny is a 700R4. I used 17 drive and 39 driven gears. This should be right on according to every calculator I used.
The speedometer is slow by a little over 35%. I checked the odometer against mile markers and it is off by 35% as well. ??????
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
I'm having some trouble with my speedo calibration too. My 80 came with a new NOS 140 MPH GM speedo in it. It only had 2,000 miles on it and I even got the original GM box it came in. The car's L82 motor had been blown up and the car sat for over 11 years.
Before I put the new motor and tranny in it I used a several different calculators on the Internet to figure out my drive and driven gears. I've got 3.07 gears and 27" tall tires, the tranny is a 700R4. I used 17 drive and 39 driven gears. This should be right on according to every calculator I used.
The speedometer is slow by a little over 35%. I checked the odometer against mile markers and it is off by 35% as well. ??????
Before I put the new motor and tranny in it I used a several different calculators on the Internet to figure out my drive and driven gears. I've got 3.07 gears and 27" tall tires, the tranny is a 700R4. I used 17 drive and 39 driven gears. This should be right on according to every calculator I used.
The speedometer is slow by a little over 35%. I checked the odometer against mile markers and it is off by 35% as well. ??????
