A quick note to the OP:
Check with your rental car agency at ORD on specifically which toll systems your rental car is set up for. Case in point... National does not use transponders but instead has Illinois Tollway charge them directly based on license plate number. They tell you to go straight through the express toll points and not worry about anything else. They also ask a question which will be critical to you: "Are you leaving the state of Illinois?" The point being is they will have to give you a transponder to put in the car if you're traveling on the Indiana Toll Road.
Lastly -- all these posts on how to avoid what might amount $5 or so on tolls when it will be just a fraction of the fuel cost???