Will speedometer readings be more or less than actual speed when the car's wheels are replaced by smaller ones? and why.
-thanks ahead of time, mike.

 

If both the wheels and tires are smaller, the speedometer reading will be higher. This is because the speedometer measures rotations rather than actual distance, and a smaller wheel/tire rotates more times per mile.

If you go to a smaller wheel but use a higher profile tire to make up for it - for example, going between the S2000 '00 16 inch wheel with '00 stock tires and the '04 17 inch wheel with '04 stock tires - the speedometer reading will not be affected.

 

Warren is correct. As long as the tire diameter is the same height you will be fine.
If I can help let me know.