I have a 1970 Chevy C-10 Shortwide, it has a 350 crate motor and it had the original trans with it. Before i switched the tranny, the speedometer was off by about 10 to 15 mph over. I now put in a B&M street and strip turbo 350 with a 2500 stall and everything works ok,except the speedometer! its off by probably 30 to 40 mph. Could someone tell me whats going on? thanks  

Sound like a major headache, does anyone have any suggestions?  

you will have to take your speedo cable loose from your transmission there should one bolt holding the piece in that it hooks to take it out and you will see a plastic gear take this to your local chevy dealer and explain your problem and they should have the right gear for your transmission.  

take it to a shop that can calibrate the speedo,,,its a simple plastic gear replacement  

I have the same problem with my 66 chevy truck. I just bought it a week ago and the spedo dos'nt work. The trans was replaced with a turbo 350. Smeone told me you need to know the rear end gear ratio, tire size, and trans ratio's. ????? I dont know where to start.