i read this today on edmunds.com....they tested the theories of how to increase gas mileage.

results here:
http://www.edmunds.com/advice/fueleconomy/articles/106842/article.html

We Test the Tips
What Really Saves Gas? And How Much?
By Philip Reed and Mike Hudson, News Editor
Email
Date Posted 11-22-2005

With gas prices so high, the media is awash with lists of gas-saving tips. Well how's this for a tip? If you listen to us, you can see hybrid-type savings without having to buy a new car.

......more..... 

I know that this is really old but I would like to make it clear that they tested the cars with low PSI in all the tires, and not only that they still had the same PSI in all tires. Tire PSI sucks gas in most cases when one or two tires are low, making the car pull to one side. Also I stress this because I have seen this a lot and most people that do not check tire PSI end up with PSI around 20 sometimes even 15 before it comes to us in the shops, that 10 to 15 PSI low. This test was only done down to 8 PSI low.