Tavhometer question
 

My 1979 Mustang Cobra had a stock 4 cyl with a turbocharger in it. As I previously stated, a friend of mine owned it prior to me and put a 1966 289 engine with a C4 automatic transmission. Does anyone know if the Tachometer's installed in these are engine specific? when I pull the dash out, on the Tach it indicated 4, 6, or 8. is this indicative of the engine (I am sure it is)? My question is; is there a method of changing this to work with an 8 cyl? Thanks.

Gary