Gas Milage Tanked in 98 STS.

Our gas mileage went from 21 miles per. gal. to barely 15 m/p/g overnight. We barely get 200 miles per tank fill up. I changed the air filter 6 months
ago. The car does not miss, there is no extra soot coming from exhaust. What could have happened?
Any help would be appreciated, TIA,
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