Fuel mileage dropped from 27 mpg to 23.1 mpg Chev. Aveo 2010

In 2016, the gas mileage was dropping from 27mpg in city to 25 mpg so
Firestone recommended the FUEL CLEANOUT REPAIR at $125.  The
mileage has dropped from 27 mpg to 23.1 mpg, 77,600 miles.  I drive only in
city, retired. I use the LUCAS FUEL TREATMENT engine cleaner one bottle
at every oil change. I use the FULL SYNTHETIC OIL 5,000 mile, never late.  
I replaced the original spark plugs Heat index 6, w/Firestone Iridium/platinum
plugs at 31, 000. Good for 60,000 miles they said. At 76,000 miles gas
dropped to 23.1 mpg, so I replaced w/new original spark plugs that came
w/my model, in the manual. 6 heat range, .35gap. Still no change in gas
miles. What else can I do to bring the car back to the 27-28 mpg?  Is there
possibly a BAD / CLOGGED FUEL FILTER.  The thermostat/housing is
brand new replaced last month & heater core both hoses replaced & radiator
cap on the overflow all new. I replace AIR FILTER every year, brand new in
OCT.