Ok I found the plate on the diff. Bunch of numbers. Here they are

DM - CN 9CA
3 - 50 9 361A

 

Is that what you needed?

 

I can't find information on exactly what all that means but it looks like 3.50 gears. If that's the case and we assume that you're fourth gear has a 1:1 ratio which it certainly should and you're running 235/75/15 tires, you should be turning 2037 RPM's at 50 MPH.

Now there is no guarantee that somebody didn't put 4:11's in there or something but still you'd only be looking at 2392 RPM's for 50 MPH. The tach is off by quite a long shot, but really we knew that already.

 

Alright thanks so much!

 

Doing a little bit of math we can figure out what would cause your tach to be so far off. For sake of conversation we will consider traditional distributor, single coil ignition systems. No coil packs, waste spark, etc.

The coil on a 4 cylinder will fire twice for every 360 degrees of crankshaft revolution.
The coil on a 6 cylinder will fire three times for every 360 degrees of crankshaft revolution.
The coil on an 8 cylinder will fire four times for every 360 degrees of crankshaft revolution.

What this means is as follows:

You have a 6 cylinder engine which will pulse 3 times per revolution. If your tach is set for a 4 cylinder, it will count every 2 as a full revolution. This means that for every revolution of your engine, the tach is seeing 1.5. As such your tach will read exactly half again the actual revs. Meaning, if you are actually turning 2000 RPM's, it would read 3000 RPM's.

If the tach were set for an eight cylinder it would be reading only 75% of actual RPM. As such, when it reads 2000 you would only be turning 1500.

So, what does this tell us. You claim to be turning 4000 RPM's at 50 MPH. The gear ratios and tire sizes tell us that you should be turning approximately 2000. Unless your tach has some setting for a 3 cylinder, which is highly unlikely, something else is incorrect.

Your tach is wrong, we know that. But I feel like something else might be wrong too because the math just doesn't work out. Perhaps your speedometer is off. This could be because somebody changed the rear end gear, or the tire size is radically different from stock, something ain't right. Have you verified your speedometer against another car or a GPS or something?

I know my '74 is actually very close to what the GPS says when running 235/75/15, this tells me that this is actually very close to the original tire size. Perhaps your truck had something different?

 

Wow... you are very knowledgable. and I will take my gps and see if my speed is on or not

 

what kind of tach are you running? i have an autometer tach that when i got it i had to take two screws apart to take the casing off and there was a little switch in there that was set on 8 cylinder and i just had to slide it to 6 cylinder so i got an accurate reading. if this is simular as to what you have i would take the two seconds and crack it open to see where it is set.

 

Its called a super pro. Its from napa

 

I tried looking for super pro tach instructions and couldn't find anything. I can find sun-pro all day long.

Ever get it figured out?