Asked and answered on here about a thousand times.......literally. Please do some searching in previous threads. Summary: You have a very old car, presumably with a LOT of miles. There could be a LOT of things causing it, most likely a combination of things. It is impossible to be much more specific than that without some more information. Reviewing the old message threads should give you an idea of what information is important to the question.
"Tune up" is like a car backfiring. Everybody thinks they sorta know what they are, but when was the last time you heard a car backfire in real life? This is because they pretty much DON'T.... Even in 20-year old cars. Same with "tune-ups" If your check engine light is not on then there are still somethings that might cause your mileage to tank, but you're going to have to start looking at some things. You might be dragging a wheel either because of a braking issue or a very low tire. You might have a dirty throttle body or mass air flow sensor. You might be using the A/C. You might be driving in a new part of the state that has a lot of hills. You might have a bad 12v battery. You might have changed the way you're obtaining your MPG figures..... We're going to need some more details. Oh....and do not take the car to a dealer and ask them to "clean your injectors" or ask if your throttle body needs to be cleaned. There's a pandemic, and a repair manager salivating all over the place at the thought of charging somebody $200 to put a $5 bottle of snake oil in the gas tank may constitute a bio-hazard. Even dealership mechanics and service writers do not deserve that..... Good Luck!
