How to check the tachomoter for correct RPM reading?

[Rustychain]

1996 Volvo-penta 4.3L GS on SX cobra drive. It seems like RPM's on tach are higher than actual rpm's. It used to run approx 30mph at 3000RPM, 40mph at 4200rpm (these are approx as it varies with load, trim, wind etc).
Now when tach shows 4200 RPM, I'm running about 30mph. Prop looks fine and no new sounds. I'm wondering if the tach could be reading higher than actual rpm. Any ideas how to check this? or is there something else I'm missing?

 

[captmello]

you need to compare using a shop tach, should be fairly easy.

 

[tpenfield]

Handheld tachometer . . . probably need to put a piece of reflective tape on the H-balancer pulley.

 

[dwco5051]

With a known accurate tach. In the olden days when accurate time was needed for navigation ships carried 3 clocks. If one showed a different reading than the other two it was assumed it was off. Installed a new tach last year on my little fishing boat, checked it against an old tach I had on the shelf and a tinytach. All three read the same within the accuracy range. Job done.