The only calibration is via the outer stop on the watchspring but that won't fix your problem, it is a very fine adjustment with a only few degrees of needle adjustment possible.
There is an "eddy current" coupler between the input shaft and the needle, the shaft rotates a magnet inside a metal cylinder causing electrical current in the cylinder which makes a magnetic field opposing the spinning magnet... and that causes a torque on the cylinder proportional to the shaft rpm: this pushes against the watchspring so the needle moves proportional to shaft rpm.
But: there is also a sleeve bearing in there mostly to keep the magnet centered. Two things can happen : the bearing lube gets gummy and the drag adds to the coupler torque, or the bushing wears and the shaft starts wobbling in it with a whining sound and a lot of added frictional torqe... this is pretty much always the cause of a falsely high reading and/or pinning tach or speedo. The hopeful fix is to wash out the bearing and apply a very light oil, clock oil is best but not so easy to find. And the coupler is devilishly hard to get at, disassembly of the mechanism is difficult without ruining the delicate watchspring. And after that, reassembly is a nightmare. Soaking the whole mechanism in acetone or other powerful solvent will clean out the bearing ... relubing is hard though. And you must remove the odometer wheels or the solvent will ruin them.
Some instruments also have a fluid damper to steady the needle... that's another kettle of fish. It dries out or leaks out (always store instruments with the face up!) and refilling them is also a challenge. But that doesn't cause crazy high readings, just a twitchy needle.
No surprise that many of the revived 60s bikes I see have new electronic instruments fitted.
