NGK iridium IX spark plugs OK or is there a better brand? on 2004 Mercury Grand Marquis

Trying to increase gas mileage-
local 9-10mpg
short hwy. - 13
long (100 miles) hwy.-25!

Asked by for the 2004 Mercury Grand Marquis
1 answer 1 comment
Use OEM for the best results. Motorcraft fortunately is available at the aftermarket stores like carquest etc...
Thank you,will do.