Low rolling resistance tires are made specially for improving fuel economy and reducing gas cost. Many car manufacturers specify the rolling resistance requirements of OEM (original equipment manufacturer) tires. These original equipment tires are specified specifically to help maximize the car's fuel economy rating. The special combination of rubber compound, hardness, size and tread pattern help minimize the tire's rolling resistance.
When replacing tires, keep rolling resistance in mind. A tire that lowers your fuel economy quickly becomes much more expensive than a cheaper tire. Using LRR tires can effect up to 4.5% of a car's fuel economy according to Greenseal.org at http://www.greenseal.org/resources/reports/CGR_tire_rollingresistance.pdf.