Can't wait to learn the answer to a really interesting question. Thanks
A P215/55R17 tire, per the labeling system is nominally 26.32 inches in diameter, neglecting the flattening that occurs in all tires. 1/4 inch of wear would reduce the diameter by 1/2 inch, or 1.9%. 1.9% of 60 MPH would be 1.13 MPH (60 indicated would be 58.87 actual ) since the distance traveled per revolution (¶d) is proportional to diameter. 59 vs 60 is almost round-off error, even with a digital speedo - wouldn't cause a ticket, especially since you are erring on the side of going slower than indicated.It doesn't take that much. The difference between a new tire and a tire nearing end of life can affect the speed readout by a few percentage points. A typical new tire has 11/32" of tread, and it's generally recommended to change the tires at 3/32". That's a difference of 1/2 inch on the overall tire diameter.