I Just Replaced My Spark Plugs And Now I Am Getting Bad Gas Mileage

I have a 1999 Mazda Protege LX 1.6L Automatic 125K. I was getting about 30-33 mpg after I replaced the spark plugs I am getting about 23-25 mpg. Now I realize that it is colder out (Wisconsin), but I don’t leave my car running to heat up b/c that doesn’t actually work. So now what do I do to improve the gas mileage. Why is it getting bad mpg?
The air filter is good, fuel filter probably needs replacing, but are there other things to do to get gas better gas mileage? Are there additives?
Please any ideas are appreciated.
I did not change the plugs b/c they looked fine. I know that they on right b/c the protege has a unique engine. I did get Bosch Platinum Plus spark plugs that do not require gaping. I did notice that it has misfires so maybe they aren’t appropriate for my car. I will go to AutoZone or Oreily’s to check it.

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