[FC] fuel gauge

[Wild8bill at aol.com]

Before I do a more indepth test and inspection, I thought I should ask the  
mentors here first.
 
I took apart my dash to replace the speedo with different (non-rusted  
face) one I had. I disconnected all wiring and only disassembled and replaced  
the speedo, but upon installation my gas gauge now does not work; stuck on  
"full". I reattached the connector and jiggled the wires (with motor on) but 
no  effect. Are there any known issues or tricks concerning this situation?
 
also, the "new" speedo seems to be 10mph slow, is that a result of the  
little spring being too tight; maybe I need to loosen it by one  revolution?
 
thanks,
 
Bill
richmond CA