Hello,
I do not totally agree with John Farley post.
A trim tab in a given position produces a hinge moment, which changes the position of the elevator (without any pilot force, and that also depends on the geared tab law).
For a given center of gravity position, and a given thrust effect (pitch torque, increase of dynamic pressure on the HTP, and change in downwash), this elevator position equilibrates in pitch the aircraft for a given lift coefficient (Cm=0). This lift coefficient, for a given weight, gives a speed.
Of course, in practical, you want to cancel the pilot effort for a given speed and attitude, and you trim the aircraft. whenever you change power or speed, you have to trim again.
chti guillaume