There are tyre / wheel sizing websites on the internet.
I haven't checked yours above.
Something to watch out for is the tyre width.
So, for example only, 255 is 1.5 inches wider than 205.
With any given offset (also important not to change) that will reduce both body to tyre clearance and chassis to tyre clearance by3/4 inch.
The wheel arch and suspension clearances (to the wheel/tyre) under dynamic conditions can alter dramatically; cannot realistically be measured.
Changing the type profile will affect your speedo, but, imho, increasing the overall diameter of the wheel tyre combination can exacerbate the clearance issue and is far more important
There are many correct and 'safe' combinations (if done properly) and even more that are not !
Lowering the tyre profile will, generally, lower the vehicle and increase ride stiffness; unless the rim is increased.
Complicated. Yes.
Which is why these tyre wheel models are so helpful in maintaining safety.
AND, to be pedantic, fitting non-'standard' tyre /wheel combinations is a Modification as far as insurance companies are concerned !
Unless changes are radical I would not expect the overall gearing to be changed.
Rolling resistance will be, so that will affect economy; I have no idea by how much. Minor, I expect.
Make sure any model you use includes changes to the clearances. They are there for a reason.