Recently bought a 54 Monterey with a 57 yblock,rebuilt trans.Does a Merc-o-metic have overdrive? I’ve driven it 50 miles and the engine is 3k rpm at 55 mph.

The Mercomatic does not have overdrive, nor does the '57 Fordomatic..

John - "The Hoosier Hurricane"

No it does not. In the "D" Shifter Position the Transmission starts in 2nd and will shift to 3rd only. Is the Transmission shifting at approx 25-30 mph with steady gas Pedal pressure? If it is not shifting the Rod from gas Pedal Linkage at the Cross Shaft may need to be adjusted(shortened).

  

re ford auto trans  57. get a book and follow the procedure to adjust the kick down, or better known throttle pressure
 if you adjust the throttle pressure on the higher sid  you will get start in 1st.
with full throttle when you leave the light trans well start in first and upshift to 2 near valve float.
i owned a 1957 merc 312  it was just a matter of throttle pressure adjustment to get 3 speeds in drive. the condition of the linkage is critical.  the rpm you quote indicates the rear axel  is to low. my mere had 3to1 rear axel(9in)

older motors repair manual had good inst on kick down adjusting. good luck