Many consumers are secretly paying more for fuel when they buy replacement tires. The tires that come with a new car are engineered to provide improved fuel economy (mileage). When consumers buy their first and later set of replacement tires, they aren't usually aware that they're paying more for gas because the costs aren't immediately perceptible and add up over time.  But your mileage might drop with these new tires. The California Energy Commission has proposed Replacement Tire Efficiency Standards that will make sure the new tires purchased are at least as efficient as the tires that came with your car.