High Tach numbers - Causes?
03-04-2003, 09:48 AM
#1
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High Tach numbers - Causes?
Awhile back I posted a question about running high tach numbers. http://forums.corvetteforum.com/zerothread?id=319157
3500rpm at 75mph. My speedo is off by 5 to 10mph so I'm really doing 65 to 70mph. The thought was big rear gears but I determined that I have the stock 3.08. What else could cause high tach numbers?
I'm redoing the rear now and thinking about 3.55 gears but with the high tach numbers already I could be getting into more trouble than it's worth..
[Modified by Koz, 9:49 AM 3/4/2003]
3500rpm at 75mph. My speedo is off by 5 to 10mph so I'm really doing 65 to 70mph. The thought was big rear gears but I determined that I have the stock 3.08. What else could cause high tach numbers?
I'm redoing the rear now and thinking about 3.55 gears but with the high tach numbers already I could be getting into more trouble than it's worth..
[Modified by Koz, 9:49 AM 3/4/2003]
03-04-2003, 10:12 AM
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Re: High Tach numbers - Causes? (Koz)
OKay, you're absolutely dead certain on the rear gears. Have you counted the ring and pinion teeth and done the math?
Perhaps it's your tach filter. You can bypass it and see what happens.
Hopefully there will be some other ideas provided here... :steering:
Perhaps it's your tach filter. You can bypass it and see what happens.
Hopefully there will be some other ideas provided here... :steering:
03-04-2003, 04:45 PM
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Re: High Tach numbers - Causes? (Koz)
Speedo off by 5-10 mph and 3500 rpm at 65-70 sure sounds like a 3.70 gear swap has been done.Mine was exactly as discribed when I installed 3.70s.Went with 200-4r and correct speedo gears and its a near perfect set up.Low end and cruisability.
03-04-2003, 05:03 PM
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Re: High Tach numbers - Causes? (J75)
Went with 200-4r and correct speedo gears and its a near perfect set up.Low end and cruisability.
03-04-2003, 07:23 PM
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Re: High Tach numbers - Causes? (Koz)
A 200-4r is a overdrive auto.Your 4 speed should have 1to1 final ratio just as my Th 400 had.I am running 255-60-15 tires.Unless you have super small tires or the clutch is slipping,I don't see those numbers being correct with 3.08 gears.Maybe you could hook up a cheap tac just to check yours.That is about the only I can think to verify.
03-05-2003, 01:38 AM
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Re: High Tach numbers - Causes? (Koz)
Koz,
If your rear gears are 3.07's, and your clutch isn't slipping massively, could be the tach circuit board. My '80 was reading anywhere from 200 to 700 RPM high. Pulled the speedo/tach cluster and replaced the tach circuit board (about $50). Tach works great now. Dead on at all speeds. Wish I would have taken this route prior to dumping the money on a new clutch. Live and learn.
Vito
If your rear gears are 3.07's, and your clutch isn't slipping massively, could be the tach circuit board. My '80 was reading anywhere from 200 to 700 RPM high. Pulled the speedo/tach cluster and replaced the tach circuit board (about $50). Tach works great now. Dead on at all speeds. Wish I would have taken this route prior to dumping the money on a new clutch. Live and learn.
Vito
03-05-2003, 10:02 AM
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Re: High Tach numbers - Causes? (av8vito)
Koz,
If your rear gears are 3.07's, and your clutch isn't slipping massively, could be the tach circuit board. My '80 was reading anywhere from 200 to 700 RPM high. Pulled the speedo/tach cluster and replaced the tach circuit board (about $50). Tach works great now. Dead on at all speeds. Wish I would have taken this route prior to dumping the money on a new clutch. Live and learn.
Vito
If your rear gears are 3.07's, and your clutch isn't slipping massively, could be the tach circuit board. My '80 was reading anywhere from 200 to 700 RPM high. Pulled the speedo/tach cluster and replaced the tach circuit board (about $50). Tach works great now. Dead on at all speeds. Wish I would have taken this route prior to dumping the money on a new clutch. Live and learn.
Vito