Suddenly poor mileage

my '96 GT suddenly started to get a lot less miles per gallon, so i talked to some poeple, they said to get a tune up, i'm actually doing that tomorrow, but does anyone know WHY my car is getting worse mileage? i kinda suspect it was one of my friends that was playing around looking at the spark plugs and not placing the cover back correctly. i checked the air filter and replaced it so that wasn't it.

any ideas would be cool, thanks.

Have the O2 sensor replaced. Because the O2 sensor reads the amount of oxygen in the fuel, when they start to go bad they will richen up the fuel mixture and cause the mileage to fall way off. I would change this or at least have the garage giving me the tune-up check it before putting a lot of new parts on the car.

cool i'll check that out, thanks