[M6-Coupe]

You have to make sure the total size of wheels be the same or very close...
The size of wheel is calculated this way:
Rim diameter + 2 side walls where the side walls are related to the size of tire.
The first number shows how wide your tire is in mm.
The second number is aspect ratio which is the ratio of side walls.
For example: 285/35/20.... the width is 285 mm or 11.22"
The side wall is 11.22 x 35/100 = 3.92"
The total wheel size = 20 + 3.92 + 3.92 = 27.84"
if the new wheel (rear rim+tire) is larger than this size your actual speed is more than what you see on speedo. If the new wheel is smaller than this size, your actual speed is less than what you see on speedo.
BTW, your speedo normally shows a bit more than actual speed