Should be 16 inch. Although some might have 15 inch, but that depends on the size of the front calipers. If the calipers are the new larger type, then 16 inch wheels.
According to the 1998 Mercury Grand Marquis Owner Guide : ( 71.9 litres / 19.0 U.S. gallons )
The 1998 Mercury Grand marquis has the 4.6 liter / 281 cubic inch Single Over Head Cam ( SOHC ) engine from the factory
Here they are: Driver = 22"Passenger = 22"
A 1998 Mercury Grand Marquis came from the factory with the 4.6 liter ( 281 cubic inch ) Single Over Head Cam , V8 engine
The Mercury Grand Marquis is a nice big car, especially those from 1986, very roomy for those who are tall, with long legs. The original tires size for this vehicle is 215/70-15. There is no other size suggestions for this vehicle.
According to the 1998 Mercury Grand Marquis Owner Guide ( and one of Fords websites ) The ( Motorcraft FL-820-S ) engine oil filter is used
6x8
15 inch
Which ones?
4x6
20 gallons
4.6l sohc v8