You guys probably know this, but for those who don't, bear in mind that changing the spark plugs doesn't just have fuel economy implications, you might land into more expensive repairs/replacements. I found out the hard way when I had to have the ignition coil replaced after driving on the remaining (still working) cylinders for over 25kms... And yet another time when the coil just quit on me, and I was told the fault was the plugs. Not good.