How does changing to 15 inch wheels affect my speedometer?

Wheel rim is 1 inch bigger but tyre sidewalls are 1.5 inches shorter so you may lose 0.5 inch which equates to about 1.7%. 60 mph speedo reading would be 59mph. However I've yet to come across a 100% accurate speedo so its with acceptable bounds for accuracy. As long as it wasn't way out in the first place.
AND the figures given on the sidewall will not necessarily reflect the true size of the tyre. As an example, my old 31x16.5R15 Hoosiers were WAAAAAAY wider than the MT Sportsman Pros of the same size. I got 27 inches for the 15s and 27.5 for the 14s.