I fitted 17" alloys to my car asI was under the impression if you fitted 18"you had to have your speedo adjusted and the turning circle clearence would be a problem. I now see ford make the c max with 18" wheels and am gutted I only have 17". I have a wheel chart and it says the offset for a c max from o3 onwards is 38 - 42 ND FOR A NEW MONDEO IT IS 38 - 45.I dont know if this will help answer your question.
c-max and focus(04-07) is 52.5mm the same as the mk3 mondeo the mk4 mondeo uses 50-55mm offset and slighly wider wheels,the same as the new(07) c-max and focus,if you have a focus or a c-max(04-07) the book says that for 18" and over you need to fit a rack limiter kit so the wheels don't rub, with 17s you should get away with it. ours runs 17s with no problems,as for ride comfort our c-max on 17s is a better ride than our mondeo was on 16s, go figure.
I`m in the market for some 17`s.Think I`ll have a good look around.A mate got some fantastic 20`s from Aus to fit his Nissan 300 a coulpe of years back,so I might try there too.I have seen a new Mondeo on 20`s & it looks awesome.
The main reason is that if you don't then they could turn round and say they won't pay out if you try and make a claim.
If the changes (of anything) are extreeme they may decale the insurance null and void just at the moment when you have to do a "producer" (you can bet"THEY'll" enquire if there are any non-standard items) amd "level 5" on the standared scale is now normally a £1000 penalty
Std steel 16" for me that you can still get for £10 cause of all the Mondeo's around
To be honest I told them in case they get robbed as the only thing people notice about my car is the wheels they are not the biggest but they are if I do say so myself very nice. I did it to protect myself against them being pinced and me having to pay out because I had not told the insurance company I had fitted them.