3000RPM @ 55MPH With 3.54 Rear

[QuickStyle]

72 GMC 402/400. My tach reads 3000 rpm while cruising at 55 mph in 3rd gear, all the online calculators show this to be accurate with a 4.56 rear end, my spid says i have a dana60 with 3.54's and i turned over the rear wheels to verify, the driveline turned 3 1/2 times for one revolution of the wheel. with this combo i should be turning 2150 rpm at 55mph.

could my tach be off by almost 1000rpm? its the factory tach and it seems to be pretty accurate by the sound of the engine and how smooth it moves. the speedometer is accurate verified by GPS. Im not sure what it could be, any help is appreciated.

[A69C10]

check your tach for any shorts or fried wires it just might need to be replaced but you are correct in the math

[RED 1970]

You didn't mention how big your tires are?

If you're running 33's then turning 3k with a 3.54 rear end means you're in second gear.

[short-t]

Did you account for converter slip?

[QuickStyle]

Originally Posted by A69C10

check your tach for any shorts or fried wires it just might need to be replaced but you are correct in the math

ya i might need to check something in there, my temp gauge is pegged at hot all the time, i removed the cluster and pushed the needle to cold, but as soon as i keyed it on it pegged hot and its stuck back there. also the volt gauge is stuck right in the middle no matter what.

Originally Posted by RED 1970

You didn't mention how big your tires are?

If you're running 33's then turning 3k with a 3.54 rear end means you're in second gear.

they are 30.5" stock size, i calculated that in also, as the online calculators ask for tire height, and its definitely in 3rd gear

Originally Posted by short-t

Did you account for converter slip?

that i did not even think of, how much is it supposed to be slipping, like is 3000rpm @ 55 mph normal? i know the 700r4's drop about 300rpm when they lockup, so i can see maybe 300rpm slipping but not 850rpm. by my calculations its just direct drive its not factoring in converter slip

[Blazer1970]

Originally Posted by QuickStyle

could my tach be off by almost 1000rpm?

Yes, it could be. Pretty much all of these old factory tachs are off by at least several hundred rpm at higher engine speeds, and they always read high.

[Longhorn Man]

stock tachs were junk 40 years ago, they are worse now.
Your amp guage probably needs new fuses.
Your temp guage probably has the green wire grounded out.

[zac]

I'm sure you've checked this already, put I thought I'd share something stupid I did years ago. I have an old, rotted out Chevy van. It has has so much noise coming at you from everywhere that you can't even think about having a conversation while riding in it. Once I noticed the fuel consumption on the interstate was even worse than usual. Going down the off ramp I noticed more engine braking than usual. So I played with the shift selector and noticed that high gear was out. And reverse. The punchline is that I had driven for over 2 hours without noticing I was cruising in second gear.

[50mercfan]

borrow an old aftermarket tach from someone. hook it up and check yours.

[69GMCLonghorn]

Originally Posted by 50mercfan

borrow an old aftermarket tach from someone. hook it up and check yours.

This is exactly what I did. I had a 4.10 rear end, but the tach was reading 3700 RPM at 60 MPH with 32's... I felt guilty about getting on the freeway! I found out my tach was off by around 1000 RPM... WHY BOTHER WITH THE THING?! I bought a new one from LMC and that one was closer (off by about 100 - 200 RPM). Once I knew where it was taching, I felt better about driving 65 - 70 without the pistons swapping holes...