Question

If you replace your tires with smaller diameter tires how does that change your speedometer reading? Why?

Answer 1

the speedometer will show higher reading then rated. This is because the speedometer actually measures revolutions of the output shaft of the transmission. For each revolution of the output shaft, the tires spin a certain fraction of a revolution.

A lsmaller diameter tire means that for each revolution of the tire, the car travels lesser than actual, because the circumference of the tire is smaller.

When you install tires which are smaller than specified by the vehicle manufacturer, the vehicle will move slower than indicated by the speedometer. This also means that the odometer will read high.

When changing tire sizes, the speedometer should be recalibrated.