a wives tale without any firm data to back it up. Check out this link to Edmunds, where they test the claims on fuel savings;
(http://www.edmunds.com/advice/fueleconomy/articles/106842/article.html)

Their conclusion: “No matter how many times drivers hear about the importance of tire pressure, most of them don’t do anything about it. They probably don’t like squatting beside their car in a busy gas station with fumes swirling around them. But is it important? The answer is yes, for a number of reasons. Properly inflated tires are less likely to fail at high speeds. They wear more evenly and, yes, they deliver better gas mileage. How much? In this test we saw a modest difference in two of the cars. It might have been more dramatic with different tires on different cars. Experts swear by it; we couldn’t really document it. And we wound up wondering if tire technology, like the design in other areas of the car, had improved.”

So with regard to your claim of up to 5% savings – maybe, if your tires were flat to begin with. And, yes, I also maintainn my cars properly and keep the tires inflated. So how much gas can I save? This is a nice anecdote, but hardly an energy policy.