I have an '82 z28 camaro with a 140 mph speedo fitted,is this original? if not whats it likely to be out of.

 

Factory 82 speedo's were 85 mph.
I would guess that it is out of a late 80's car. If I'm not mistaken, the speedo's in the 91-92 cars used a pick-up in the trans to signal the speedo.

 

gm (i believe it was) later let out a new speedometer to replace many of the z28's later on. most likely the person who had it before you just bought this aftermarket one and put it in. I did that with the 140mph your talking about cuz my old speedo broke (I was fooling around with the dash) heh heh heh.