I have a 98 WS (and an 87 Bronco II). My WS gets about 19 mpg mixed around-town and highway driving. Plugs are double platinum (BTW, difficult to change rear plugs), oil is Mobil-1 synthetic, engine tune is good. Note that gas mileage is determined from calculations which record every drop of gas put into the vehicle over extended mileage intervals, i.e. VERY accurate.
My WS will approach 22-24 mpg only with pure extended highway driving at ~60 mph speeds. Conversely, expect less than 19 mpg for pure city/town driving. The 3.8L engine is peppy, and once you start getting leadfoot-ish, mileage drops quickly. My WS has about 200,000 miles, and runs well. Transmission has been replaced once (POS). Electrical gremlins are NUMEROUS and are a continual pain-in-the-ass. Door sensors, locks, windows, interior lights all a mess. Most of my light bulbs have been removed and door lock fuses pulled. Still can't figure a way to disconnect the $#!&*^%$! door sensors.
Also, 1999 and later 3.8L engines will likely develop a "Mixture Too Lean" vacuum leak problem requiring an expensive fix to replace intake plenum isolator bolts, due to use of an inappropriate plastic intake manifold material.
There are probably better vehicles to buy than this one. Just my opinion.
CraigK