On your car, a tune-up consists of replacing the spark plugs. It's also
recommended to replace the air filter. But unless your car is misfiring
or you've badly neglected your air filter, I wouldn't expect either of
these to significantly affect your fuel economy.
I'm no expert on fuels, but if the gasoline was reformulated in your area
within the past year, this could be one cause. I am now noticing 10%
ethanol stickers on fuel pumps in my area. I don't recall seeing that a
year or two ago.
Here's what I'd recommend checking:
-- make sure tires are fully inflated
-- check your tire wear to see if it indicates a need for alignment
-- if your check engine lamp is on, diagnose the trouble codes
-- if automatic, check to make sure your car engages all four gears.
-- if automatic, also check to see if the torque converter clutch (TCC)
engages at steady highway speeds on level ground. You may have difficulty
doing this. The best advice I can give is to check when you're on a
reasonably flat highway with little traffic. Once the torque converter
clutch engages, lifting or depressing the accelerator pedal slightly will
not make the rpm change. The transmission control module regulates when
the TCC engages, so it may take a significant amount of practice to learn
how to get it to engage.
I think that's about the limit of what you'll be able to do yourself.