A bigger tire has an increased circumference. For every rotation, the vehicle travels further. But it still registers the same on your dash, because the mileage shown is based on the number of wheel rotations. When it says you've traveled 1 mile, you may actually have traveled 1.2 miles. You may think you re getting, for example, 18 miles per gallon, when you are actually getting 22 miles per gallon. I suggest you find a measured mile, drive it at exactly 60 MPH with a stop watch running, and see how many seconds you are off a minute when you complete the distance. Don't start from a standing start, enter the mile at 60 MPH. It should take you exactly one minute. Bigger tires throw out your speedometer reading as well as your odometer reading.