Here are the rules for Illinois
“You must drive on the right half of the roadway except:
* when passing another vehicle moving in the same direction on a two-lane highway, interstate highway or controlled freeway.”
In Wisconsin:
State “keep right” laws
You may use the left lane (when there is more than one lane in your direction) to pass. You may or may not be able to use the left lane when not passing. The table below describes the law in effect in each state.
A few states permit use of the left lane only for passing or turning left. These have “yes” in the “keep right” column. Six states require drivers to move right if they are blocking traffic in the left lane. Most states follow the Uniform Vehicle Code and require drivers to keep right if they are going slower than the normal speed of traffic (regardless of the speed limit; see below). These are listed as “slower”, with an asterisk and an explanation under “comments” if vehicles lawfully using the left lane must yield to overtaking traffic. A few states either do not require vehicles to keep right (“no”), or permit vehicles moving at the speed limit to drive in the left lane regardless of traffic conditions (”<SL”).
The color coding in the “keep right” column is red if the state has no restriction on slow vehicles in the left lane, yellow if vehicles moving at the normal speed of traffic are permitted in the left lane even when they are unnecessarily obstructing other traffic, green if use of the left lane is limited to passing, and grey otherwise.
Wisconsin slower 346.05(3) (Note: this is yellow on the chart.)