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?

06-02-2005, 04:51 PM	#1
Chevy Muscle < 69 Blazer Join Date: Mar 2004 Location: Nevada, TX Posts: 292	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?



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