[Olympiadis]

I just did a test of changing nothing but 4-tires.
Depending on the types of corrections used for the data, the lowest measured gain is 6.3%. Others may see a little larger gain depending on how you run your test and the types of correction factors used.

Keep in mind this was a swap from four old worn Douglas radials to four brand new tires. All testing was done at 50 PSI, and highway test loops were used at a speed of 55 MPH using cruise-control. Conditions monitored and data collected with two Scangauge2's, and a Davis weather station computer, with final fuel usage determined at the pump at the end of each loop. I have 4.5 years of data collection on this car previously and many of the same type test loops performed in the same way and in the same location for this car and several other vehicles. The testing error range on this particular car is a little under 1%, so I don't feel the need to run back-up tests on this, nor would I remount the old tires for a test.

The Michelins are significantly heavier than the Douglas tires.
Size for both sets is 195/60-15 (stock for Ford Focus)
Douglas tire = 15.8 lbs each
Michelin tire = 21.8 lbs each

The Michelins have a very deep tread ( 9mm ) for such a small tire, and are quite stiff, which I'm sure accounts for the extra weight. They seem much better made all around than the Douglas brand.
They ( Michelins from Tirerack ) cost around $100 more per set than Wal-mart's cheapest, but are supposed to perform really well in wet conditions, and I consider the safety factor worth it even without the mileage improvement, so I'm obviously satisfied with this choice in tire.

Depending on corrections applied the test results ranged from a low of
+6.3% to a high of +6.8% MPG.

Again this was old vs new tires with a significant difference in weight change involved. It's not exactly what you could call a fair test between the two tire types, but the vehicle did achieve a new high in MPG for this test loop, so indications are that these tires do perform as they say they do.
I'll have to update my vehicle data when I get some free time.