Hey,

I'm not really sure which forum to put this in so i'l start it here.

My car is an 84 z28 with a 5speed tranny. This weekend my speedo decided to quit so i went to the pick n pull and got another instrument cluster from an 88 berlinta(i think). The original cluster in my car had kms on one side and mph on the other (where it went up to 140 kph). The one i got from the junkyard has kms that go up to 180. After installing the "new" cluster the speedo starts at 20kms and when i'm driving about 50km it says i'm going around 90. The tach also says its revving higher than it actually is(i'm assuming this is from the speedo). I used the speedo wire from the km/mph speedo from my car. I obtained the speedo wire from the junkyard car but never put it on. Could the problem be in the speedo wire, or possible in the gears in the speedo? Thanks for the help.

 

One possibility is the "stock" rear end from the donor car was a different size than the one in your car. I changed rears in my 89 IROC from a 2.77 (I think) to a 3.42, the speedometer is now off by about 8 to 10 MPH. When I'm going 50MPH, the speedo reads about 60MPH. The speedometers are calibrated from the factory for the specific rears that they came stock with.

 

If you change rears then your speedometer will be off, but he only changed the speedometer so the tranny still has the correct gear for the rearend. I replaced a 110mph with a 145mph and it still worked fine. So there must be another reason for the discrepancy.

 

I don't know why it isn't working right.
The speedo drive gears in the tranny are what makes up for different rear end gears.

 

Not sure about the speedo, but I believe they are all supposed to be calibrated the same. I think the spec is like 1 cable revolution per second @ 60MPH (assuming you have the right gears in the trans output shaft that drive the speedo, which you should since you haven't changed anything down there). Don't quote me on the number, just realize there is a common GM spec that I beleive has been held constant for a very long time.

Tach..... did that gague cluster come out of a V6 car? If so, it'll definitely read too high in a V8 car. Tachs are calibrated differently for engines with different numbers of cylinders.

 

Alright thanks for the help. It was from a 6 cylinder car so i guess thats the story of the tach.

I'm begining to wonder if maby the speedo is screwed.