Get the compression test done from any good workshop, you will get to know the condition of the Pistons, rings, valve seals by the reading! Lower readings means lesser power and lower fuel average.
Use Shell 10W40 if the car isnt eating oil.
Change the Spark Plugs and Spark Plug Wires (HT Leads)
Get the Timing checked by a professional slightly retarded or advanced can cause fuel average to change.
same as with the air fuel mixture, get the carburetor serviced and adjusted.
Tyre Pressure is fine. are the tyres factory spec?