Tach Vs. Hobbs Time

Hello

Im trying to figure out the Tach on our clubs Archer is screwing us. I
just flew down to the Centenial of Flight FFA from Planville CT. Round
trip with all diversions and ATC rerouts was 975nm. The tach said 12.4
hr. I never checked the Hobbs but I do know that for this plane the Tach
usually runs faster than the hobbs on cross countries. Is this normal. I
also did another flight previously at 65% power from Meriden MMK to
Williamsburg W94 in 4.6 Tach time. My watch said 3.9. Also based on the
4.6 my fuel burn was only 8 GPH.

Is there any text book way to check this. Our FBO seems to feel that
calibrating the Tach is a big deal. I'm currently thinking of just
replacing the Tach without tring to calibrate. Right now it looks to me
as if we just replaced and engine at 1700 hr thinking it had 2000 hr.

Thoughts ideas?