Assuming the car knows the circumference of the wheel, it calculates the mpg from the number of rotations per gallon. Now, since the circumference is greater with the new tire, the same number of rotations should cover a fractionally greater distance. So the same amount of gallon takes you slightly farther. Consequently, the mileage should actually *increase*.

What am I doing wrong?

 

The car does NOT know the circumference, it uses a fixed value to multiply the # of rotation sensed to get what it THINKS the distance traveled.

When tires are bald, the # of rotations per mile increases, this gives the car false sense of greater distance traveled for a given amount of gas. It just multiply a pre-determined circumference to this increase # of rotation sensed.

This is the exact reason when you install an aftermarket tire/rim package, RX's odometer and speedometer are off. So is Nav.

 

Mr_Rx300,

I totally trust my local costco. This is the second set of tires I had done there. They are very courteous and professional. They use a torque wrench like they are supposed to. I trust them more than any other tire place I've ever gone to.

I hope all the costco's are run like the one by my house.