70 Z Gear ratio or Tach wrong?

1970_Z28_503

Veteran Member
Apr 12, 2017
132
Oregon
So new to me 70 Z28.

Original M22 4 speed and 12 bolt rear end marked date correct 3.73 Posi rear end running 225 60/r15 tires.

I have been chasing a rear vibration problem that I thought was a bent wheel as I am getting it only from 72-85 indicated MPH. Right rear wheel is a touch out of round but the vibration goes away when I push clutch in coasting down hill at same speed so I figure the wheel isn't the issue. Now I say indicated because my GPS tells me the Speedo is as wrong as the day is long. (55 MPH GPS = 70 speedo) Anyway at 55 MPH GPS, my tach is reading 3300 RPM and that trend continues at about 5-600 RPM higher than what it should be reading for 3.73 gears as I go up in speed @ 75 MPH GPS I am reading around 4400 RPM.

Based on the calculator I have been using it says i have a around a 4.56 gear ratio. That would indeed explain away my 15 MPH speedo issue i guess and the crazy high RPMs right?

Well here is the Rub, when I put the car up on the rack the tech marked the shaft and tire and did the old spin test he said it kept coming back at right around where it should at 3.73.

Question I have is there a logical reason my Speedo and Tach would be wrong or is the Tech just not reading the marks correctly and there are 4.56 gears in that rear end? Does anyone know if 1970 Z28 12 bolt 8.875 rearend that came factory with 3.73 gears use the style 3 or 4 housing and if it is 3 is it even possible to get 4.56 without changing said housing?

I am popping the rear cover on the 12 bolt Monday to look around and trouble shoot that vibration and will count teeth and all that jazz, Just wanted to see if anyone here had a better guess as to what could be happening.
 

FlaJunkie

Veteran Member
Lifetime Gold Member
Mar 24, 2001
6,454
Rockledge, Florida
Possibly have the wrong speedo gear in the transmission. You can easily remove it and inspect it.

By the way, initially my 3.73 speed was double the tach. 3500 was 70, etc.
 

Gary S

Administrator
Lifetime Gold Member
Apr 14, 1999
24,419
Bismarck, North Dakota
Those early tachs usually read high so don't rely on the tach. The speedometer should be accurate if nothing has been changed. The 225-60 tires are smaller than the original F60 tires were. A 235-60 or even possibly a 245-60 should be closer in size depending on the tire manufacturer.
 

FlaJunkie

Veteran Member
Lifetime Gold Member
Mar 24, 2001
6,454
Rockledge, Florida
Mine are all 235-60-15 and my tach is still off.
I really suspect our tachometers 'age' and there is probably some things that could use a checkup. Like checking any caps in the device...or measuring the resistance between the tach and the source. I'm thinking I may get a refresh of mine along with an oil gauge to replace the ammeter.

netcamoil.jpg
 

1970_Z28_503

Veteran Member
Apr 12, 2017
132
Oregon
I've have MSI ignition so I thought that might have something to do with it. Guess I will see what's up Monday, Going to look at the transmission speedo deal and see if that looks bad. Once I know for sure what gear ratio I am looking at i think my course of action on troubleshooting will be clearer.

What's it take to change Ratios on a Check 12 bolt? So much freeway time around here I am thinking 3.42 might be nice..
 

Dave Nelson

Veteran Member
Dec 19, 2003
5,811
Vineland N.J USA
I verified the accuracy of my tach by using a old engine analyzer tach/dwell meter placed on the cowl, and sitting in the car I compared the readings with my original tach, they were exactly the same.
 

1970_Z28_503

Veteran Member
Apr 12, 2017
132
Oregon
3.73's right? guess the tac is off 600 rpm give or take. Looks like i Need new pinion bearing and Ujoint though.
 

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