On http://www.tirerack.com we read – freely translated:
„"In the United States, vehicle manufacturers are required to maintain an average fuel consumption for the fleet of new vehicles they sell each year. In 2009, the government mandated the Corporate Average Fuel Economy (CAFE) target of 27.5 miles per mandate gallon (mpg) for passenger cars and 23.1 mpg for light commercial vehicles (including minivans, vans, pickup trucks, and sport utility vehicles). However, because it is an average fuel consumption target, in order to sell larger cars or trucks (which also mean higher fuel consumption), the vehicle manufacturer must also offer smaller cars and trucks that are more fuel-efficient. The vehicle manufacturer can be penalized if their annual fleet consumption exceeds the target.".
A tire's rolling resistance affects fuel consumption. Most vehicle manufacturers require their suppliers to develop low rolling resistance tires for original equipment fitting on their new vehicles. This is intended to improve the average fuel consumption (MPG). To meet the manufacturers' requirements, these tires are often designed with a priority on reducing weight and rolling resistance, and feature thinner sidewalls. Flatter treads, shallower profiles, and tread compounds that generate lower rolling resistance are also targeted.
However, to understand CAFE tests and the role tires play, it's important to recognize that CAFE tests are conducted in a laboratory, not on the highway. Many factors that affect fuel consumption in the real world don't necessarily adhere to the laboratory constants specified in the formulas.“
Therefore, practical help is available: Retrofit TireMoni.