I have been driving extremely conservative to increase mileage.
Like JoeyWhat stated, a $100.00 add on will take a Looooooong time to recoup your cost and even longer to see the benefit.
Read Elizabeth's post about driving habits.
I was driving a 2002 Dodge B1500 at 12MPG...No matter how I drove, the best was about 14MPG. My gas cost is about 1/3 of the Dodge with the Focus.
2012 5 Door Hatch SE
203A & Sport package with 16" wheels