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09-24-2012, 12:19 PM
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#1 |
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Registered User
Join Date: Aug 2011
Location: Gallatin, TN
Posts: 782
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Tach question
1977 C10, GEN II LT1-350, TH350 trans, 3:08 gears. 295/50/20 rear tires. At 75mph ( per my GPS) my tach shows 3900 RPM. I really don't think this is correct. Any ideas?
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77 C10, 6.0LS/4L80E, 3:73 12 bolt. Boss 338's 20" x 10" w/ 285/50/20 rear. 20" x 8.5" w/ 245/40/20 front. Vintage Air. Full Autometer gauges recessed in factory bezel (Thanks bigblock73!) Gm Storm Grey Metallic paint "Cool ain't ever cheap and cheap ain't ever cool" |
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09-24-2012, 12:59 PM
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#2 |
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Registered User
Join Date: Aug 2010
Location: Olathe, KS
Posts: 61
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Re: Tach question
You sure about your gear ratio? AT that RPM you would normally have a 4:88. If everything is as you say, then your tach has gone south or you're stuck in second gear.
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1982 C-20, 454, TH-400, PS, A/C, 71,000 miles 1978 C-20, 350, TH-400, PS, Non-A/C, 112,000 miles If it ain't broke, there is still probably room for improvement.  Speed costs money, how fast do you want to go? |
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09-24-2012, 01:09 PM
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#3 |
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Registered User
Join Date: Aug 2011
Location: Gallatin, TN
Posts: 782
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Re: Tach question
I am positive about the gear, took the cover off and counted the teeth myself. Also, I know it is in 3rd gear, so I am guessing my tach if off. Autometer says you can calibrate them and they are sending me instructions on how to do it. Now I have to come up with something to compare it to so I can calibrate it. Might be cheaper and easier just to get a new tach...
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77 C10, 6.0LS/4L80E, 3:73 12 bolt. Boss 338's 20" x 10" w/ 285/50/20 rear. 20" x 8.5" w/ 245/40/20 front. Vintage Air. Full Autometer gauges recessed in factory bezel (Thanks bigblock73!) Gm Storm Grey Metallic paint "Cool ain't ever cheap and cheap ain't ever cool" |
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09-24-2012, 08:29 PM
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#4 |
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Registered User
Join Date: Dec 2011
Location: Regina, SK, Canada
Posts: 622
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Re: Tach question
I had exactly the same issue and posted in these forums for help so I can't take full credit for this. But I did follow this link and presto all fixed!!
http://howtoalmanac.com/kevin/projec...chfixtruck.htm I just follow the proceedure and all was fixed.
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1979 Chevy Dually Crew 4X4 454, D60 Tru-trac, D70 Detroit Locker, 3.73, 4L80E/NP205 1971 GMC 3/4 Ton 4X4 |
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09-24-2012, 09:43 PM
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#5 |
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Registered User
Join Date: Aug 2011
Location: Gallatin, TN
Posts: 782
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Re: Tach question
Thanks for the info, bilfman. I have an Autometer tach so this will not work forme. I appreciate the effort though!
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77 C10, 6.0LS/4L80E, 3:73 12 bolt. Boss 338's 20" x 10" w/ 285/50/20 rear. 20" x 8.5" w/ 245/40/20 front. Vintage Air. Full Autometer gauges recessed in factory bezel (Thanks bigblock73!) Gm Storm Grey Metallic paint "Cool ain't ever cheap and cheap ain't ever cool" |
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09-24-2012, 09:51 PM
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#6 |
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Registered User
Join Date: Nov 2010
Location: Windsor Locks
Posts: 628
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Re: Tach question
Dumb question but is it set to 8cyl mode?
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09-25-2012, 03:36 PM
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#7 |
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78K & 79C Jimmys
Join Date: Nov 2009
Location: Ottawa Ont CANADA
Posts: 7,901
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Re: Tach question
was wondering about the setup too for 8 cyl
if you cut those little loop wires it makes it for 4 or 6 orange/brown
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John 1978 GMCJimmy4X4-350/203 1979 GMCJimmy4X2-305/350 |
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Linear Mode
