With a 4.6 L engine, on the hi-way, you should get around 28 MPG. Have the injectors checked, timing, and sensors.
For a 1999 Mercury Grand Marquis : ( 15 city / 22 highway ) miles per U.S. gallon , according to a website I was looking at
The ( 2007 ) Mercury Grand Marquis has a ( 19 U.S. gallon tank )
( 20 U.S. gallons )
According to my Chilton's Auto Repair manual : A 1988 Mercury Grand Marquis has a ( 20 U.S. gallon ) gas tank
Mercury Grand Marquis was created in 1983.
A 2001 Mercury Grand Marquis will hold approximately 20 gallons of gasoline. The fuel economy is 17-25 miles per gallon.
According to my Chilton's Auto Repair manual : A ( 1987 ) Mercury Grand Marquis has a ( 20 U.S. gallon gas tank )
For a 1999 Mercury Grand Marquis : ( 15 city / 22 highway miles per U.S. gallon ) according to a website I was looking at
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.054 is the gap for the Mercury Grand Marquis.
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According to a website I was looking at : For a 2005 Mercury Grand Marquis : ( 16 city / 23 highway / miles per U.S. gallon )