The less aggressive the tread pattern, the better the mileage.
One thing that WILL affect your mileage is by putting on oversize tires, this will cause a drastic reduction in perceived mileage. Taller tires mean less rotations per mile, also shows less miles on your Odometer. A good tire dealer can show you how a calculation that will compensate for the size difference.
For Example, my Chevy Silverado has larger tires on, but I have to add 3% to the mileage to accurately figure my gas mileage...
