Assuming it’s a mechanical driven tach, try lubing the tach cable. Make sure it’s not bound or pinched. Clean it and the casing, then lube the cable. If that doesn’t work they’re not costly. Replace the cable. I think I paid after tax $33 for the one on my 97 Tercel from the dealer.
Sometimes the teeth on the gear wear out we’re the tach goes into on top of the transmission, or the gear that it mates to inside the transmission. Other than that it’s the tach itself. Most of the time it’s the cable.