Rally tyre size codes

[Bryn7]

Why are rally/competition tyres coded differently to road tyres and what do the codes mean. eg. I am told that my present rally car tyres (Pirelli 165/55-13R road tyres) are the same sizes as Kuhmo 160/570-13. I am looking for wider tyres and these seem narrower?
Can anyone explain how the figures are interpreted?
Or recommend a wide competition tyre that will fit my 13" X 5.5" Revolution alloy wheels.
Thanks

[sfaulds]

No 2 manufacturers measure width the same (in my experience), so you really need to see them side by side.

The 'profile' figure is different because road cars have an 'aspect ratio' relative to the width marked on them, whereas competition tyres have a diameter (mm).