Speedometer off
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7th Gear
Joined: May 2019
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Speedometer off
Hello, I have a 1986 Corvette that the speedometer is off. The faster you go the further off it gets, IE: at 30mph the speedometer shows 34 but at 65 it shows 84. I don't know if the gears have been changed or not, but i do know that the car was raced on road courses by the previous owners. Would the calibration tool i see on some sites fix this problem, or do i need a new cluster? Any help would be appreciated. Thanks, Charlie
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Hello, I have a 1986 Corvette that the speedometer is off. The faster you go the further off it gets, IE: at 30mph the speedometer shows 34 but at 65 it shows 84. I don't know if the gears have been changed or not, but i do know that the car was raced on road courses by the previous owners. Would the calibration tool i see on some sites fix this problem, or do i need a new cluster? Any help would be appreciated. Thanks, Charlie
It could be corrected maybe using just drive/driven gears or maybe actually a ERA device could be less expensive and a better/less expensive option.
Last edited by WVZR-1; Feb 14, 2020 at 12:13 PM.
Melting Slicks
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From: Question Government
I always thought that only wheel & tire height/size could change the speedometer accuracy. If nothing else works, try this online site for any instrument cluster/ gauge repair or uprade: www.taninautoelectronix.com
Last edited by RetroGuy; Feb 14, 2020 at 12:23 PM.
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While you're checking take notice if the VSS is a center connection and it's color maybe. Could be black or white best I recall for an early car. It could also have numerical references on it. Look for maybe 35 - 39 or 40 - 45. It could also have maybe a connector on the outside diameter, important to know which and all information that's on it.
Last edited by WVZR-1; Feb 14, 2020 at 12:26 PM.
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The 30 MPH error is a 12% error.
The 65 MPH error is a 33% error.
This is not a VSS gears or ratio problem because the error is not linear. Nor can both errors be corrected with a VSS ratio change. A calibrator will not help either, as it applies a constant correction to all speeds. I think you have a cluster problem. Seems like a component has aged and is affecting calibration. Probably an electrolytic capacitor in the oscillator circuit used as a time-base to calculate speed and rpm.
The 65 MPH error is a 33% error.
This is not a VSS gears or ratio problem because the error is not linear. Nor can both errors be corrected with a VSS ratio change. A calibrator will not help either, as it applies a constant correction to all speeds. I think you have a cluster problem. Seems like a component has aged and is affecting calibration. Probably an electrolytic capacitor in the oscillator circuit used as a time-base to calculate speed and rpm.
.....................
OP - I didn't see the 'spread' IHBD mentions. I saw maybe 13 (rounded) and less than 30. I had a friend that I could send on the mission five times to compare and he'd seldom return with matching info.Depending upon the accuracy of the GPS (I imagine that's your comparison device) do a drive using 4 or 5 reference points and then compare. Do it maybe a few times.
Last edited by WVZR-1; Feb 15, 2020 at 11:14 AM.
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Try Greg @BlowerWorks for a 25513046 - that should be brown BUT you do need a correct VSS
Safety Car
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From: Minneapolis Mn
I realize this is an old thread, but some of this may apply to others. This is just a thought to confirm where the OP is regarding the speedometer being off. I had an issue with my 86 when I bought it with the speedometer being off. To figure out where it was, I used a GPS unit in the car while driving to get a true speed. Set the cruise at various speeds, noted the speedometer reading along with the speed shown on the GPS. My solution won't apply to the OP as there was a Dakota Digital converter wired into the speedometer to convert the speed signal from the Tremec transmission to be compatible with the car and there's an adjustment on the converter to alter the speed signal. In my case I was able to align the speedometer to the true speed via the adjustment on the converter.
Here's a question though. I would think that if the speed shown was off due to tires and/or rear gear change, wouldn't the amount off be linear? Meaning if it's off by 4 mph at 30 then shouldn't it be off by 8 mph at 60? I think the amount of actual inaccuracy in the speedometer needs to be nailed down before a solution can be pursued.
Just some thoughts.
Here's a question though. I would think that if the speed shown was off due to tires and/or rear gear change, wouldn't the amount off be linear? Meaning if it's off by 4 mph at 30 then shouldn't it be off by 8 mph at 60? I think the amount of actual inaccuracy in the speedometer needs to be nailed down before a solution can be pursued.
Just some thoughts.
Last edited by hcbph; Jun 20, 2021 at 07:17 AM.
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Here's a question though. I would think that if the speed shown was off due to tires and/or rear gear change, wouldn't the amount off be linear? Meaning if it's off by 4 mph at 30 then shouldn't it be off by 8 mph at 60? I think the amount of actual inaccuracy in the speedometer needs to be nailed down before a solution can be pursued.
Just some thoughts.
Just some thoughts.
The OP never returned with his information. It could be assumed he figured it out.
@ihatebarkingdogs once again returned to delete his comments the same day he posted so it could be assumed his information was 'speculatioon' based!
Last edited by WVZR-1; Jun 20, 2021 at 07:58 AM.