I'm looking at an SDV6 SE at an independent used car dealer. The vehicle presents well, and has been listed as having the 20" black pack, but upon closer inspection I noticed the wheels appear to be off a range rover, and are shod with 275/45R20 Coopers. I suspect these are not the standard wheels for the factory 20" pack. Also, the spare underneath is a standard 19".

Is there any problem running wheels of this size (mechanically, legally, insurance etc). Are there any advantages of these over the stock 19's/20's?

This concern has potentially put me off pursuing the vehicle at this stage, as I wonder what else is lurking under there I haven't seen...