depends on the overall diameter, the wheel size is irrelevant. If the overall diameter increases the speedo will read lower and if the overall diameter decreases the speedo will read higher.
How much...well it's not an exact number, it will be a ratio. I don't have any idea what the formula to figure this out is. Although, with a measuring tape and some general info about your car you could prolly come up with it if you spent some time on it.