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Speedometer and tachometer

Tiny
WIZZERD02
  • MEMBER
  • GMC
I have a 1986 cutlass salon and recently I had the alternator checked out at auto zone, wherein they require me to rev the engine to 3000 rpm. However, when I rev to 3000 on the tachometer, the autozone machine read only 2000. Which is more likely to be correct, my tachometer or their machine? Also it has seemed that I get passed on the highway a lot even when doing ten over the speed limit, how can I test my speedometer accurately?
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Thursday, April 13th, 2006 AT 6:39 PM

2 Replies

Tiny
BURRBRIAN
  • MECHANIC
  • 158 POSTS
Do you have an aftermarket tach like a SUN tach or something like that? If you put it in yourself did you check and see if it has a switch for 4-6-8 cyl? You might have it on 6 cyl. If you are reading that much higher. Just a thought, if it's a factory tach I'm out of ideas. On your speedo have you changed the rear tires to a different size than what the factory put out on it? This will affect your reading. Smaller tires will run speedo faster than you are going- bigger will make it read slower. Hope this gives you something to work with. Good luck. :
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Friday, April 14th, 2006 AT 8:27 AM
Tiny
WIZZERD02
  • MEMBER
  • 1 POST
It has stock tach and speedometer. Tires are factory.
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Friday, April 14th, 2006 AT 3:29 PM

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