Michelin XDA Energy or Bridgestone M713 drives?

[JimT]

It's pretty well known that stiffer tires have lower rolling resistance than soft tires, but it's also a balance between rolling resistance, grip/traction, ride comfort and durability. The lower the tire pressure the more the tire flexes as it rolls around. That extra flex takes away some of the rotational energy and converts it to heat. Lowering the pressure will help smooth out the ride a little and may improve traction slightly, but it will result in lower fuel mileage and more heat buildup in the tires - maybe not enough to cause a failure, but enough to notice a difference in fuel. You see the effects of differing pressures very clearly in racing applications where slight changes of even 1 psi can have drastic effects. Personally I try to keep my tires within +/- 5 psi of the max inflation as long as they are all equally pressurized.