A tach will tell you a lot of things. First off, your engine manual will tell you the optimum rpm's for your engine, which you should never try to exceed. Then, by adjusting your prop pitch, the angle of the engine to the transom and height of the engine (ideally the cavitation plate should line up with the deepest - lowest - centerline of the transom), you can get the highest mph (use a GPS) and performance out of your rig. It's called "tricking out" your rig. That also improves fuel economy as well. Win-Win.