With engine oil filter change
The 4.0 L - V6 - takes ( 5 quarts - U.S. of 5W-30 )
The 4.6 L - V8 - takes ( 6 quarts of 5W-20 )
According to the 2004 Mercury Mountaineer Owner Guide
According to the 2004 Mercury Mountaineer Owner Guide : ( 22.5 U.S. gallons )
According to the 1997 Mercury Mountaineer owners manual : Your 4R70W automatic transmission capacity is ( 13.9 quarts ) * that of couse would be for the transmission , torque converter , lines , transmission cooler , etcetera *
The 4.0 L - SOHC - V6 engine that is used in the Mercury Mountaineer takes : 5 quarts ( 4.7 liters ) with oil filter change
22.5 gallons
You don't mention the model year of Mercury Mountaineer , or the engine size , but the 4.0 liter Single Over Head Cam , V6 engine has always taken ( 4.7 liters / 5.0 U.S. quarts of 5W-30 with engine oil filter change )
According to the 2007 Mercury Mountaineer Owner Guide : ( the fuel tank capacity is 22.5 U.S. gallons )
According to the 2000 Mercury Mountaineer Owner Guide : ( the gas tank holds 21.0 U.S. gallons )
22.5 U.S. gallons
4.5 quarts
7 US quarts
79 liters or 20.86 gallons.
I believe it is ( 20.8 U.S. gallons )