In our series on Gas $$ Savings Tips, I mentioned putting nitrogen in your tires.
To put my money where my mouth was, I decided to do this on one of our vehicles. It cost me $21, but it is going to be worth it. I put the nitrogen in a vehicle (a 2005 Chrysler Pacifica) that usually got about 18.5 - 19 miles per gallon (and we made sure the tires were inflated). On our first tank with nitrogen, I got right at 20 miles per gallon.
That may not be a huge difference, but, over the course of the next few months, we will easily see a difference in our mileage and gas cost.
Some with smaller vehicles are getting much better differences (percentage-wise) when they make the change. And, keep in mind, nitrogen keeps your tires inflated longer, which makes a big difference on a long trip.