Tach Accuracy

[Already Gone]

Is there any way you can tell if a factory tach is giving accuate reading ? Is it possible to have the tach not reading properly when the speedometer is, as verified by a GPS, dead on accuate ?
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[463]

Hi Greg: one of these will work when the hood is up http://www.ebay.com/itm/ES-125-Timin...-/200793775899
or wire in a 2nd tach.

[TBONE1964]

Absolutely, like 463 says, use a timing light that has a reading for RPM. They are pretty accurate.

[Already Gone]

okay, so you hook up the timing light under the hood and take it into the truck hooked up and watch the dash tach? This my sound like a dumb question but is there enough cable on the timing light to go from under the hod to inside the cab?
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[TBONE1964]

Originally Posted by Already Gone

okay, so you hook up the timing light under the hood and take it into the truck hooked up and watch the dash tach? This my sound like a dumb question but is there enough cable on the timing light to go from under the hod to inside the cab?
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Actually a very good question and you have a point. Probably not enough cable to do that. If you have another set of eyes to help you, that would work. You under the hood with the timing light holding the throttle at 2000 and your helper inside watching the tach.

You might also have enough cable if you run the conductor on the #7 plug wire and use power from inside the cab for hot and ground on the timing light. Maybe a modified cigarette liter adapter or something near the fuse box.

[Already Gone]

Bump for more, if any, ideas.
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[Lee H]

My factory tach was off in varying degrees across the whole range. I spot checked it like Tbone mentioned. After it failed I had it rebuilt with VDO internals and it seems to be real close but have not verified it. I'm thinking the tachs are like the other gauges, about, in the ball park.

[Already Gone]

I am asking this because I bought a 12 bolt with posi ( which it has ) and was supposed to have 3:08's in it and I didn't pull the cover to check the numbers on the gears before I bought or installed it. I got it installed the other day and took it out for run to check it out. Its quiet and the posi is there but at 65mph ( verified by my GPS, my speedo is right on ) with my 255/70 15 tires I'm doing 2800RPM which equates to a 3:73 ratio and that is what I had. I would be very surprised if the guy I bought the rear end from lied or tried to decieve me. Hence that is why I asking the tach question.

[463]

use this link http://www.huntergraham.com/twcc.htm

[Already Gone]

Originally Posted by 463

use this link http://www.huntergraham.com/twcc.htm

Thanks for the link Trueman. I've checked my specs on a similar calculator and that is where I discovered that with 3:73's, 255/70 15 tires and a 3 speed auto trans at 65mph I'm doing 2800rpm and with 3:08's it should be 2300 rpm. That is 20% less which you should give me 2 or 3 more miles per gallon on the highway.

[PatrickGMC70]

Interesting question, but in your situation, it's probably easier to just jack up the rear end and see how many times the rear tires rotate for each turn of the drive shaft. Three times means you have the right gear ratio; 3 3/4 means you have the 3.73s.
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[71K20chevy]

If the tach isn't reading correctly, take a small flathead screwdriver and reach up behind the dash. On the back of the tach is a screw covered with tape. Poke through the tape and adjust the screw until the tach reads correctly. Of course that is only for a set RPM, as it's been said above that it loses its accuracy once it goes up through the ranges.

[Already Gone]

I appreciate the suggestions but I have figured out 100 % that my tach is out. Last week I replaced my complete differential for one with posi. I had the rear end that was removed from the truck in my garage and checked the numbers stamped into the housing and it decoded to a 72 with a 3:07 ratio. I then removed the cover and checked the numbers on the gears and they decoded to a 3:07 which is what truck would have camewith from the factory and the date code stamp is earlier than the build date on my truck so I'm sure the rear end I removed from the truck was the original. Now when that rear end was in the truck the tach also showed 2800 RPM at 65 mph which tells me the tach is reading wrong. Again thanks everyone for the suggestions.
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