Milage down on Explorer

Have Had a Ford explorer for long time Its a 1993.Always got 20miles to the gallon ,Lately the mileage has dropped to 12-14 and the temp gauge seems to run low .Put in a new thermostat
but gauge still seems low ,Getting fair heat but not like before seems like the pressure in the radiator is not their ,Have no engine light warnings .Have not changed wires or oxygen sensors ,Where to start is the question ?
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