It's a 2.5% difference in speedo.

The only other thing to look out for is any change in offset - the Mondeo wheels might have a different offset to the winter steels. It basically affects how the wheels 'hang' off the suspension, and can introduce too much camber, depending on numbers.

You could try here www.willtheyfit.com/#isPage=1

You should find the dimensions of wheels including offset stamped on the inside of the wheel, PCD and bore have to be the same (you can alter bore smaller with spigot adapters if necessary but PCD is either right or wrong) of course you still have to offer a wheel up on the car to make sure its going to clear brakes and suspension etc and don't forget to put the car at full steering lock too.