How does changing to 15 inch wheels affect my speedometer?

for instance a 205/70-14 and a 205/65-15 are almost exactly the same circumference at 79.6" for the 14", and 80" for the 15", height is 25.4" for the 14" and 25.5" for the 15", so the 15" wheel will mean you are going .6 MPH faster at 60 MPH than the 14" wheel