This question is asked loads of time on the forum. When you change your wheels/tyres your speedometer will show an incorrect reading. This little calculator will show you what difference it will make to your readout. REMEMBER: Your speedo will not be accurate in the first place.
Select the tyre size you currently have, and then the size you're interested in. Calculate each tyre size and then click on the click to calculate the difference button. It will show you all the rolling radii, circumferences, percentage differences and even speedometer error.