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Old 01-21-2010, 11:00 PM   #1
TwoFiftyShifter
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Aftermarket Tach reads a little odd

Just installed an Auto Guage autometer in my '87. It does this weird thing where it reads a different rpm when the truck is cold then when hot. Not much, but noticeable. When cold 2000 rpms is 60mph, when warm, 2000rpms is about 55mph. Tried it a few different times and it still does it. Any idea why? Is that normal?
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Old 01-21-2010, 11:18 PM   #2
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Re: Aftermarket Tach reads a little odd

the truck runs higher when cold
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Old 01-22-2010, 09:34 AM   #3
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Re: Aftermarket Tach reads a little odd

Sure the mechanical speedometer isn't the source of the incorrect readings?
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Old 01-22-2010, 10:09 AM   #4
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Re: Aftermarket Tach reads a little odd

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Originally Posted by TwoFiftyShifter View Post
Just installed an Auto Guage autometer in my '87. It does this weird thing where it reads a different rpm when the truck is cold then when hot. Not much, but noticeable. When cold 2000 rpms is 60mph, when warm, 2000rpms is about 55mph. Tried it a few different times and it still does it. Any idea why? Is that normal?
Maybe you have some clutch slippage when it warms up. Mine kind of dried out and worked better when cold but when the engine warmed up and leaked oil more easily it got the clutch oiled up again and it started slipping.

High gear acceleration is the place you will start to notice a warn clutch first.

That was the first thing I had to do to my K20 was yank the engine and reseal it all so the new clutch would last and work right.
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Old 01-23-2010, 10:37 PM   #5
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Re: Aftermarket Tach reads a little odd

Hey guys, did a trip today, and with the use of my new GPS, i have come to the conclusion that its the speedometer. It reads about 10mph fast when cold, 5mph when hot.
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Old 01-24-2010, 01:41 AM   #6
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Re: Aftermarket Tach reads a little odd

never heard of a speedo being faster or slower when hot or cold.... weird
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Old 01-24-2010, 02:02 AM   #7
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Re: Aftermarket Tach reads a little odd

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never heard of a speedo being faster or slower when hot or cold.... weird
Not really. It is cable operated, and is lubricated by either grease or transmission fluid. Difference in lubrication temps means the difference in the resistance for the shaft running the speedometer.

I have had trucks do that, and have had ones that will have a jumpy speedometer when they were warmed up too.
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Old 01-24-2010, 02:10 AM   #8
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Re: Aftermarket Tach reads a little odd

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Not really. It is cable operated, and is lubricated by either grease or transmission fluid. Difference in lubrication temps means the difference in the resistance for the shaft running the speedometer.

I have had trucks do that, and have had ones that will have a jumpy speedometer when they were warmed up too.
I guess that makes sense. Maybe never noticed the speed difference while going down the road... 5mph isnt that much i guess to notice going down the highway.
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