The two external sources of resistance to your car's motion are air drag and tire friction. Using low rolling resistance tires can improve your gas mileage by up to 5%. In the past tires with lower rolling resistance usually wore out really quickly or had poor traction, especially on wet roads. Having to replace tires often will defeat any savings on gas with the increased tire cost. However, modern day technology is now able to make low rolling resistance tires without compromising wear or traction. One of the key innovations has been to reduce the weight of the tires and increase the stiffness only in the areas that flex the most.
Tire Rack did a series of tests on new generation tires. I was interested to notice that they used the ScanGauge II in the tests. Tests covered fuel economy, ride comfort, noise comfort, road handling, wet and dry braking and wet and dry cornering. The conclusion? Modern low rolling resistance tires will save on gas without giving up on handling, wear or comfort.