Speedometer?!?!

I have a 1978 El Camino, with a 350 & 350 turbo tranny out of a 1971 Camero Z28. I bought the car and on the way home, the speedometer broke- just went to zero, and wouldnt move. So I got a new sp. cable, and the auto parts store said it was the same cable for a 71 Camero, and a 78 El Camino. So I brought the cable home and made sure it fit in the back of my 78 speedometer. It did, but in the process of putting it on, i snapped off the needle. So that same day an 81 El Camino Speeometer was on ebay. It said that they all had the same gears from 78-87 so I figured it would work. So i got it all installed, and the speedometer is not accurate. And its not just like 10mph off- when i go 25 it says 10, when i go 45 it says 35, when i go 60 it says 52. So its a % off.

So my real question is- Is a speedometer calabrateable, or am I screwed? I dont even know if it was accurate with the original (78) speedometer.

You can either buy a speedo adapter from jegs, or Sumit racing, or go to a speedometer shop and for a few bucks you can buy the small plastic gear thats inserted into your tranny where the cable connects.

graymarki