Tachometer Question.......

[ts71281]

I bought a restored instrument cluster from a member on here (who I will not name) with a camaro tach installed. Other than the bezel being in 40 million pieces from no packing materiel and the bezel being a 67 with a choke hole (which I asked about) and the printed circuit board that had corroded and de-laminated, the rest of the cluster works. However when idling at 850 the tach only reads about 500-550 RPMS. I'm pretty sure I got the wrong tach and that it's for a 6 cylinder. Can someone please give me some positive insight to this and tell me this tach can be converted to a 8 cylinder tach easily? I have more money in this stupid instrument cluster than if I bought a new one from LMC!! Is there some kind of resistor I could take out of another 8 cyl gm tach (I have lots of oldsmobile ones laying around) and just swap it over. It seems all across the whole tach it's always about 400 RPM's slow. Anyone ever just bend the needle? I'm just trying to get by cheap is all. Thanks guys!

Joe