Chicago is one of my favorite cities, with lots of amazing things to do. And once you're in the city, you really don't need a car--the el, buses, and taxis will get you almost anywhere you want to go, relatively cheaply. As a tourist, honestly, it's probably better not to have a car. Parking can be pretty dire and very expensive, especially downtown.
But the key to not needing a car is "once you're in the city." It sounds like where you're staying is pretty far out (40 miles or so). That's sort of like saying "We're going to London!" when you're actually going to Slough. In traffic, the drive into/out of the city is probably going to take more than an hour. It's doable, definitely, but you're probably going to end up spending a lot more time in the suburbs than in the city itself.