They are adjustable.




They also have a weird wiring setup. I know I had one working fine on a '75 240 electronic ignition system in a 122. From what I remember, the tach ran on the slow side, but adjusted using a cheap aftermarket digital tach, and it matched up fine.

Paul

Here's a description on the wiring. I know the color was different on mine. If you need more info, let me know and I can figure out how it's setup on mine. (Mine has 3 white and 2 black wires coming out of the tach, and has the complete wiring harness intact).

https://www.brickboard.com/RWD/volvo/1426810/120-130/123gt_smith_tach_wiring.html

The 1967 123GT came with a tachometer which had P/N281046 manufactured by Smiths for Volvo. The 123GT tach differs somewhat from the Smiths tach that was later sold as an accessory in that it has a chrome ring and a 10 x 100RPM grading. The accessory tach had a matte ring and a 1 x 1000RPM grading. Both had the same part number. It had 5 wires, one black, two red and two white. Wired as follows;
Black wire to ground
One red wire to instrument lighting
One red wire to the 25A fuse
One white wire is connected on the coil iso the black which is removed.
One white wire is connected to the distributor (where the just removed black wire was).
If the tach doesn't work, switch the two white wires.

Suggest and look inside the tach to see if the black wires replaced the red wires and if the single white wire replaced the black ground wire.
Hope this helps.