http://www.ehow.com/guide_7-http://www.ehow.com/guide_7-cars.html.htmlwww.ehow.comhttp://www.ehow.com/how_2118573_better-mpg-properly-inflated-tires.htmlYour tires are more important to fuel economy than you might realize. In fact, keeping them aired up at the proper pressure will definitely save you money. The Environmental Protection Agency says that cars driven on tires with lower air pressure than specified can cost up to 3 percent in fuel efficiency. Improperly inflated tires create greater rolling resistance, which means it takes more energy to move your vehicle down the road. You can remedy this with just a few minutes of tire care each week. Published on 11/11/2007http://www.ehow.com/tips_2118573.html There is no scientific basis for the use of nitrogen in car tires.<br/> Consumer Reports magazine tested tires filled with nitrogen, and found that they retained pressure slightly better than with air, however, the difference was only 1.3 pounds per square inch over an entire year?s time ? a trivial difference which may even have been an accidental result.<br/>In addition, the Rubber Manufacturers Association advises that, ?For normal tire service applications, nitrogen inflation is not required.?<br/> Normal air is 4/5 nitrogen, so replacing it with "pure" nitrogen is pointless.<br/>