Tach reading way too high ? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Chevy Muscle · 06-02-2005, 04:51 PM

I bought a tack/air pressure dash from a swap meet early this year. It came from a big truck and there is an idiot light on the fuel gage for 'tandem axle'.

I replaced the air pressure gauge with a vaccume gauge and put a normal bezel on it and put it in my blazer. I hooked the tach up to my HEI (tach spade).

The probem is that the RPM reading is WAY too high for what the engine must really be turning. I have a TH350, NP205 , 3.07 gears and 33" tires and it was reading close to 4000 rpm at 70mph (if I remember right).

Why is my tach reading way off like that? It returns to 0 when off and reads about 700rpm at idle (but I'm guessing it really idleing around 500rpm).

Could I have a 6-cyl tach? If that could be the problem did 6-cyls come in big trucks with tandem axles?

blazerparts · 06-02-2005, 06:14 PM

most early "big" trucks were 6 cyl

longhorn71 · 06-02-2005, 06:35 PM

most i have seen here are 8cyl

**jamie72** · 06-02-2005, 06:45 PM

it will be stamped on the back of the tack 6cyl or 8 cyl. and there is a screw on the back also, it for adjusting the tack reading, try playing with it to see if you can get it to read correct, but first i would hook up a know good tack to check the one you got.
jamie

**pjmoreland** · 06-02-2005, 07:02 PM

Engine RPM = Gear Ratio x Speed x Transmission Ratio / Tire Diameter / .002975

Engine RPM = 3.07 x 70 x 1 / 33 / .002975

Engine RPM = 2188

You're right. It is reading way, way too high. If this is being caused because it's a 6-cylinder tach, then it should be reading 2188 x 8 /6 = 2917 RPM, which is still quite a bit lower than what you're seeing. Something else must be wrong.

Longhorn Man · 06-02-2005, 07:07 PM

yeah, stock tachs sucked 30 years ago.

Chevy Muscle · 06-03-2005, 09:03 AM

Quote:

Originally Posted by pjmoreland

Engine RPM = Gear Ratio x Speed x Transmission Ratio / Tire Diameter / .002975

Engine RPM = 3.07 x 70 x 1 / 33 / .002975

Engine RPM = 2188

You're right. It is reading way, way too high. If this is being caused because it's a 6-cylinder tach, then it should be reading 2188 x 8 /6 = 2917 RPM, which is still quite a bit lower than what you're seeing. Something else must be wrong.

I may be remembering wrong about the 4000rpm. I haven't been up to 70 in a while and I've slept since then. I do remember it was way too high and my calculations at the time lead me to believe that the rpms would be pretty close if it were a 6-cyl. I guess I'll have to test it again and I'll post up.

I'll pull it out and see what the tach says on the back. If it is indeed a 6-cyl, how hard is it to replace it? (if I have to buy a new tach to put in this, then I'll have too much money in the darn thing

I could'a done better buying one from someone here.

)

71swb4x4 · 06-03-2005, 01:19 PM

Even if it is a 6 cyl tach you may have enough adjustment with the screw to get if pretty close.

06-02-2005, 04:51 PM	#1
Chevy Muscle < 69 Blazer Join Date: Mar 2004 Location: Nevada, TX Posts: 293	Tach reading way too high ? I bought a tack/air pressure dash from a swap meet early this year. It came from a big truck and there is an idiot light on the fuel gage for 'tandem axle'. I replaced the air pressure gauge with a vaccume gauge and put a normal bezel on it and put it in my blazer. I hooked the tach up to my HEI (tach spade). The probem is that the RPM reading is WAY too high for what the engine must really be turning. I have a TH350, NP205 , 3.07 gears and 33" tires and it was reading close to 4000 rpm at 70mph (if I remember right). Why is my tach reading way off like that? It returns to 0 when off and reads about 700rpm at idle (but I'm guessing it really idleing around 500rpm). Could I have a 6-cyl tach? If that could be the problem did 6-cyls come in big trucks with tandem axles?

06-02-2005, 06:14 PM	#2
blazerparts No,I DON'T have Tourett's Join Date: Mar 2005 Location: Reno, NV Posts: 556	most early "big" trucks were 6 cyl __________________ '72 K5 CST 350/350 Daily Driver '69 GMC3500 Camper Special CST Flatbed 396/400 Upgraded to show truck '69 GMC 2500 L6292, 3spd beater

06-02-2005, 06:45 PM	#4
jamie72 Senior Member Join Date: Apr 2000 Location: madison indiana Posts: 3,839	it will be stamped on the back of the tack 6cyl or 8 cyl. and there is a screw on the back also, it for adjusting the tack reading, try playing with it to see if you can get it to read correct, but first i would hook up a know good tack to check the one you got. jamie __________________ jamie72 72 BB cheyenne -- Restoration getting started.. member #807 http://www.myspace.com/whitetailobsession www.antlertalk.com

06-03-2005, 01:19 PM	#8
71swb4x4 Senior Member Join Date: Nov 2003 Location: Brookings, SD Posts: 10,497	Even if it is a 6 cyl tach you may have enough adjustment with the screw to get if pretty close. __________________ Some people are like slinkies, they aren't good for anything, but you can't help but smile when you see one tumble down the stairs.

06-02-2005, 06:35 PM	#3
longhorn71 Senior Member Join Date: Sep 2004 Location: CANADA Posts: 1,681	most i have seen here are 8cyl

06-02-2005, 07:02 PM	#5
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	Engine RPM = Gear Ratio x Speed x Transmission Ratio / Tire Diameter / .002975 Engine RPM = 3.07 x 70 x 1 / 33 / .002975 Engine RPM = 2188 You're right. It is reading way, way too high. If this is being caused because it's a 6-cylinder tach, then it should be reading 2188 x 8 /6 = 2917 RPM, which is still quite a bit lower than what you're seeing. Something else must be wrong.

06-02-2005, 07:07 PM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	yeah, stock tachs sucked 30 years ago.



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