Last year I took off my Focus 18 inch titanium spec alloy wheels and fitted some 16 inch steel wheels with winter tyres. I had to buy second hand steel wheel nuts as I could not get the new wheel plate covers on with the existing nuts, they stuck out too much. Just as a foot note when I took my winter tyres off after last winter, I re fitted them with 16 inch normal road tyres. The roads being so pot holed these days and having bent an alloy on a previous car, I found the ride is so much more comfortable, cheaper tyres and more importantly it appears to have an improved the MPG.
I believe the rolling radius has stayed very similar as the GPS on the Tom Tom it still gives the Focus speedo as 5 mph higher than the GPS.