Here's a question. My Lincoln also has 245/40R19 - 26.7" diameter factory tires on it now and I'm planning on 18" snow wheels/tires.
Probably going with a 18x8" wide rim, thus looking for snows in the 3 possible sizes:
245/45R18 26.7" <-- factory dia. but sorta wide for snow
235/45R18 26.3" <-- 1/2" lower than factory
225/50R18 26.9" <-- best choice for snow, but MKZ is a heavy car
Narrower tires are actually better in snow, but I'm worried the 225 will be a bit narrow on an 8" rim.