Very odd!

Make sure the springs and struts are not stiffer aftermarket ones.

I assume your tire pressures are not too high.

Plugs will not increase gas milage. Bad ones will hurt it. Ford double platinums are reccomended.

Other problems can hurt gas mileage - EGR, O2 sensors, dirty MAF...

"Those who cannot remember the past are condemned to repeat it" -George Santayana