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More details please. Year, engine, tranny, was the cluster swapped in in place of a non-tach cluster? I know for a fact that if you swap in, oh, let's say a Exploder Sport Trac cluster into about a 95-97 Ranger and you don't do the proper wiring tweaks, the tach sticks at about 3500 RPMS since a cluster light (or thereabouts) is what is wired into the swap-in tach cluster...but, I digress.
It will not affect performance, but in some cases can indeed be a troubleshooting tool...