Terrible mileage 99 GLE

Anybody suggest why?

I only put about 4K on the car since I bought it 1-3/4 years ago with
92K on it.  I mostly drive around town, but also take the freeway to
"blow it out".

Lately I notice the needle dropping like lead. I can't afford to go
anywhere, with these gas prices!  

I took it back to CarMax and they did what they call a tune-up,
which is just replacing the spark plugs.  Is that a tune-up?
They *did* put in platinum plugs, supposedly better?

I also had them replace air filter, fuel filter and change the oil.

$$$$ and mileage is no better.

What else could be keeping mileage so low?

Tx

Aspasia