Do the tyres rub on anything? I considered 245 width tyres last September but decided not to get them as I thought the tyre was too close to the shock absorber. I imagine there isn't much of a gap with 255's, maybe your car has spacers fitted...worth checking?
-are they standard (well 19s would have been an option) wheels? if someone has gone aftermarket they could have an odd value of offset. Otherwise as frembrit says they could have spacers.
- is there any sign of rubbing. Check the tyres all over for signs of odd wear partic around the shoulders and the side walls. Also listen for odd noses as the suspension compresses and extends.
- Does it look to be on standard springs. Someone who has changed the tyre size may have also gone for eg lowering springs. This probably isn't a big problem if they've used decent parts but if they've just cut the existing springs that could be a concern.
Depending on what has been changed you'll probably have to tell your insurance and they'll probably want cash from you.
I would be very concerned for one major reason.
If the car is not fitted with original spec 18inch or optional 19 inch wheels and corresponding Ford approved size tyres as per the hand book spec etc, your insurance company could refuse a claim and void the policy.
As per policy terms etc you should advise of ANY modifications or changes.
If/when you tell the insurance company of the non standard changes they will either refuse cover or charge you an additional premium