IIRC, a battery that drops and holds at 12.2V is about 50% of rated capacity. You can get by with it until you need to "dig deep" and need the extra capacity. A Chicago winter might induce that need...
Check that the alternator is charging it, too. It should be over 14V running and not exceed 15 revved. But at that age, a new battery is probably wise before it gets much colder. I have found good deals the last few times I needed a battery at Firestone. Buy online, decline the installation, and the price for a very good Interstate is better than most deals you can find.