An informal survey of football fans show Michigan Wolverines fans outnumber Michigan State fans (plus every other team) in the vast majority of the state.
You probably already knew that, but it's pretty cool to see on a map:
State fans are only a majority, per this Reddit poll of college football fans, in a few counties in western Michigan and the upper peninsula. Michigan territory also bleeds into the upper corners of Ohio and Indiana.
There were 3,462 responses to the Reddit survey, which respondents coming from 787 counties.