I have one too , but it's good to see DATSUN bros. givin parts too each other . Thats what you call lookin out for each other to keep them dats . on the road
I have a 72 coupe and it didn;t have a tach, instead it had a clock. But clock is gone now and aftermarket tach sets in it's place. So I don't know about the options .
Get a police officer to test your spped with a radar . I f you yalk to them right they will be glad to do this. then record this info with your tachometer.