Haha yeah, with mine they not only got the wheels but also took the front grill and the boot spoiler! There was also another fiesta + an Audi and a BMW. Fortunately they didn't do any physical damage to the cars.
Right ok, i'll see if I can get some tyres that are a little big larger. I'll pop into my local garage tomorrow and see what they say.
Its very simple -
The larger diameter wheels have lower profile tyres (eg the ST wheels/tyres)
The smaller diameter wheels have higher profile tyres
so the outside diameter stays the same/ similar
So the outside dianeter of the wheel/ tyre combination stays similar, whatever the wheel size so the ride height stays the same, the speedo does not go out (by much) and the cars gearing is not changed
BUT - if you fit higher profile tyres on the larger diameter tyre the outside diameter of the tyre will be larger, the car will sit higher, the gearing will be higher (you may even need to change down more on hills etc) the bigger tyre may rub on the wheelarches, the speedo will read lower, so you will be going faster at the same indicated speed
there are charts to work this all out, but if you decide to go ahead with the higher- profile tyres - it would be wise to check the actual speed against the speedos' indicated speed with a GPS- based device with an accurate speed readout - as the deviation may already be on the edge of its permitted amount
The other thing is you must buy 4 wheels together, you can never mix different outside diameter tyres on the same axle, and its bad practice to have different (outside) sizes front-to-back (and would defeat the purpose of what you are trying to do) - so effectively, you would have to throw thour old, low profile tyres away (i suppose you could sell them on if they were decent)
Another point is as the Fiesta ST wheels are designed for a certain width/ profile of tyre, fitting non- standard ones may invalidate your insurance, (they will already know you have the ST wheels, of course, as a modification)