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Is this normal? Tach/spedo

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Old 09-13-2017, 11:20 AM   #1
Joe Madden
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Default Is this normal? Tach/spedo

5500 rpm @81mph have c6 staggered 18/19 wheels, stock 3.07 gears.
This is at 68mph, think its an electrical or mechanical issue? 90 stock l98
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Old 09-13-2017, 12:05 PM   #2
WVZR-1
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With the KEY-ON engine NOT running does the tach park @ 0? If it doesn't then I'm quite sure you've a calibration issue. If the car is a C68 ATC you can get the RPM that the ECM sees by using the diagnostics in the C68 control. I'd get a scanner on it to confirm if you could.

Without actual tire dimensions it's impossible to tell you what to expect for RPM at any MPH and actually with the tire/wheel swap it's certainly not correct on the speedometer UNLESS it's been corrected for the change.

Have you ever compared the speedo/odo to a GPS for validation or error?

Last edited by WVZR-1; 09-13-2017 at 12:06 PM.
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Old 09-13-2017, 12:57 PM   #3
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Im pretty sure the speed is correct, i have done the gps thing and im within 1.5 mph at 60 mph. I am at zero with key on but at idle im well over 1k but under 2k and it idles normal. Just blows my mind that its in redline and im definitely not revving that high. I know WELL what 5500 rpm feels like. I appreciate the response, ill check the c68. Im familiar with using it. My temp gage dont work either lol.
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Old 09-14-2017, 12:13 PM   #4
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90 and 91 clusters are prone for being brutal.

My tach reads off by 200 rpms at idle, reads 2400 rmps at 2000, etc. I've verified with TTS and the HVAC method.

My oil temp gauge doesn't work, but works on TTS. IT shows some movement as it gets hot but is way off. Oil pressure is pretty accurate as well as coolant temp and volt meter. Just remember those gauges are not linear...

I am looking to get mine rebuilt. Not sure where yet.
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