As I said, its usually NOT that much difference with regards to overall diameter. 24.1 inch is, however, the accepted diameter to shoot for or be close to. I use 215/45-17's on my ZX3. When we got them over five years ago there wasn't much of an after market wheel selection available and since my son wanted to buy the wheels (SVT argent Euros) I let him go for it. The downside of bigger wheels and tires is more weight resulting in more rotating mass because this weight is now farther from the hub. More rotating mass results in slower acceleration, longer stopping distances, and makes the suspension work harder. It can be very noticeable especially in low powered and moderately braked cars like the Focus. If I were to do it over I would select some lightweight 16's (yes hard to find but they're out there) and some lighter performance tires (maybe even some 15's if i could find the right tires). I acknowledge there is often a trade off between looks and outright performance. I personally prefer the latter. Please note that the two charts provided above have inconsistencies with regards to overall diameter for identical sized tires. Not by much but there are differences.