Cadillac: 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,


Terry