Your speedo will register out. It'll be such a % slower. I don't know the maths to work it out (because I don't understand tyre profiles), but someone here probably does.
You also won't get the same acceleration as you would on 13" wheels, because of rotational mass etc etc, but you can achieve a higher top speed! Although I dunno if the difference will be noticeable, and you shouldn't really be getting to the micra's top speed on public roads, where you'd be using bigger wheels.