I'm a little late to the conversation, sorry bout that.
Any loan in Illinois that is secured by a car title specifically does not meet the definition of a payday loan, and therefore does not have to follow the PDL laws.
However the lender should have a CILA license and be regulated as such. If they aren't licensed, then their lien on the car could be avoidable. I would be hard-pressed to say that any storefront is operating without a license, though. These types of installment loans generally are a simple interest method, which means that interest does accrue every day that you have a balance. For the first 1/3 of the loan, almost all payments are going to interest and very little to principal. Delinquency can also play a BIG factor -- make a payment even 10 days late and there the account has earned an extra 10 days interest that comes out of your payment ....... if you ever get to be 30 days or more delinquent, you will basically owe so much interest that your payments will never touch the principal. And in Illinois the simple interest method is completely legal.
Others have given very good advice, and I can't really say much different. Your goal will be to get it paid off, I think, rather than fight the legality of it. If they are licensed, their calculations are probably correct even though you don't like it.
If you do want to go the legal route, here may be a tip ... most title loans don't actually perfect their lien with the secretary of state. Technically they are supposed to send the title to SOS with an corrected title application, to record their lien. That costs $65. The title stores usually just hold onto the title without actually recording the lien information through SOS. Therefore, when their lien is not perfected, they really are not entitled to repossess a vehicle if they have not dotted their i's and crossed their t's.
You could check with the Secretary of State to see if they have actually recorded the lien. If not, and they repo, you may have legal recourse against them.