You are correct, they are all electronic these days, however, they are still counting the revolutions of the transmission gearing. Those revolutions were based on the diameter of the tire the vehicle came from the factory with. When you change the diameter of the tires, the number of revolutions in the trans does not change, but the distance the vehicle moves per revolution is different. The difference between what the computer believes the vehicle travels because of a tire diameter, and the amount the vehicle travels with a changed tire diameter gives you an incorrect speedometer reading. once the computer is informed (reprogrammed to know) of the different tire diameter, the reading will be correct again.

In the old days, it was done with a gear change in the speedometer drive, these days its done by telling the computer what has changed. Gene