Does anyone get worse gas mileage after replacing tires? 2012 X3 -Put Primacy MXM4 RunFlats instead of Pirellis back on ---same size, all four new tires $280 each and now my gas mileage is 3-4 miles per gallon worse. Tire store had never heard of this but came up with this answer: Michelins had 11/32 tread compared to 7/32 with brand new Pirelli's so the computer reads the resistance differently. I wanted a mileage tire instead of the Pirellis which only lasted 17K miles with mild driving habits. Help!?!?