Speedometer Calibration
Thread Starter
Pro
Joined: Mar 2001
Posts: 587
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From: Leonard, MI USA
Speedometer Calibration
I have a '67 convertible. According to the officer that pulled me over for speeding, it looks like my speedometer is reading off by about 10mph at 50mph. Meaning I am travelling 60mph, but the speedometer says I am travelling 50mph. Is there anything I can check myself to correct this? Also, if I have to take it to a speedometer shop, any good ones in the southeastern Michigan area? Thanks in advance.
Advanced
Joined: Jan 2002
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St. Jude Donor '10-'11-'12-'13-'14-'15
Having the same problem with a V6 MR2, easiest thing I can think of is to borrow a friend's GPS, they're pretty doggone accurate, next best is a stopwatch and the interstate mile markers... Paul
Burning Brakes
Joined: Jul 2003
Posts: 786
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From: Emporia, pay no attention to that man behind the curtain, KS
If your speedometer seems to working well other than being off, you can change the driven gear at the transmission. Since the speedo is slow, you will need to go to a gear with fewer teeth. I would suggest you pull out the gear, count the teeth, and start with a gear one or two teeth smaller.
You could also install an adapter at the transmission that will correct the speedometer. This will screw on in place of the speedometer cable and then the cable screws onto the adapter. You should be able to find one for under one hundred dollars. I'm sure that will be less than taking it to a speedometer shop.
You could also install an adapter at the transmission that will correct the speedometer. This will screw on in place of the speedometer cable and then the cable screws onto the adapter. You should be able to find one for under one hundred dollars. I'm sure that will be less than taking it to a speedometer shop.
Melting Slicks
Joined: Jul 2004
Posts: 2,865
Likes: 195
From: Maryland
If you know the rear end gear ratio and tire diameter of your car you can compute your speed . For a 205-75-15 tire (27.1") and 3.36 rear end gear
in 4th gear you are doing about 50.3 MPH @ 2100 RPM.
http://www.angelfire.com/fl/procrastination/rear.html
You can also follow a known accurate speedometer equipted car and see whar revs your turning at a known steady speed.
in 4th gear you are doing about 50.3 MPH @ 2100 RPM.
http://www.angelfire.com/fl/procrastination/rear.html
You can also follow a known accurate speedometer equipted car and see whar revs your turning at a known steady speed.
Team Owner
Joined: Oct 2000
Posts: 38,897
Likes: 1,926
From: Washington Michigan
Originally Posted by kyosho
Also, if I have to take it to a speedometer shop, any good ones in the southeastern Michigan area? Thanks in advance.
http://www.specmo.com/
Instructor
Joined: Mar 2005
Posts: 163
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From: Roswell GA
Speedometers (and tachs) are magnetized, and after 40 years, they will loose some of the energy causing them to read low. If you are handy, there are places that sell precalibrated speedo assemblies so you can simply swap it out, otherwise I would recommend sending it to a Corvette specific instrument rebuilder and have it rebuilt and restored.
Thread Starter
Pro
Joined: Mar 2001
Posts: 587
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From: Leonard, MI USA
Thanks for all the replies. I have checked it against some others and I know that it is off. I think I'll take JohnZ's advice and take it to spemco. Now that he mentions it, I remember the name of that place from way back.
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Melting Slicks
Joined: Jan 2003
Posts: 2,817
Likes: 21
From: Somerset, NJ
Originally Posted by Trophy Blue
I tested mine when I saw the police radar trailer parked in front of my house. I found out that my speedometer was off by 4 mph.
Le Mans Master
Joined: Feb 2006
Posts: 6,048
Likes: 5
From: "cutesy" TN
Originally Posted by csalisbury
Speedometers (and tachs) are magnetized, and after 40 years, they will loose some of the energy causing them to read low. If you are handy, there are places that sell precalibrated speedo assemblies so you can simply swap it out, otherwise I would recommend sending it to a Corvette specific instrument rebuilder and have it rebuilt and restored.
Race Director
Joined: Apr 1999
Posts: 10,866
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From: Beverly Hills/Pine Ridge Florida
Originally Posted by knight37128
I did not know there are magnets in them. I always thought they were mechanical.
BUTI've seen speedo's calibrated at Corvette Spec. of Md. Our Chapter has had a few tech sessions about this.
Brian has a large machine, a "gauss-degauss" machine. Using the machine, he can impart a certain degree of magnetism to the 2 spinning disks. You can also "de-gauss" the disks and remove the magnetism.
The two spinning disks act like a "slipping clutch". By imparting the correct degree of magnetism to the disks, they spin at the proper rate.
It can take a number of tries to get the magnetism correct.
This calibrates the speedo head correctly, and has nothing to do with using the correct driven gear in the trans. Chuck
Burning Brakes
Joined: Jul 2003
Posts: 868
Likes: 5
Speedometers are magnetized and can loose a bit...not common but it can happen.You'll see the contact points wear and they don't read as they should more than loosing the magnetic pull.
The speed reading is the result of a magnetized worm gear spinning and slighty pulls the speed cup with a hair spring.
Also on that gear is a worm ...it drives two other gears that turn the ODOMETER.
Speedos are calibrated in a 1:1 ratio ....the colored gears in the trans are the adjustment for rear end and tire ratios to make it read correctly for the set up you have.
Jason
The speed reading is the result of a magnetized worm gear spinning and slighty pulls the speed cup with a hair spring.
Also on that gear is a worm ...it drives two other gears that turn the ODOMETER.
Speedos are calibrated in a 1:1 ratio ....the colored gears in the trans are the adjustment for rear end and tire ratios to make it read correctly for the set up you have.
Jason
Melting Slicks
Joined: Feb 2002
Posts: 2,500
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From: Severn Maryland
St. Jude Donor '05-'10-'11-'12-'13-'14-'15
first - what size tires are you running? are they the OEM-size, or not? second - are you still running the same rear-end and tranny as original?
both of these will impact what you are seeing on the speedo
both of these will impact what you are seeing on the speedo
1967, 2001, c2, c3, calabrating, calibration, corvette, fast, magnetization, magnetize, marlboro, md, mechanical, showing, speedometer, tools, upper
