Miles per gallon?

In no particular order,and some/most already mentioned;
SpeedO error
Insufficient ignition advance,faulty vacuum advance,too-tight valves,low compression,fat carb
bad convertor,restrictive exhaust
engine running too cold;cold fluids and greases, ie winter running
flat tires,dragging brakes,gears
long warm-ups,short trips,stop and go,high speeds
driving style,hard into it,
winter