Auto Loan Wizard

Unlike a mortgage, which uses Monthly interest and can be calculated using the Loan Wizard, your auto loan almost certainly uses DAILY interest when the amount of interest owed this month depends in part upon what DAY you made last month's payment.

Q has no way of knowing what day your lender posted that payment.

SO, the only thing that you can do is, when you get the next statement, is to go back and edit the prior payment to conform with what the lender shows on that statement.

" It doesn't seem to have a register for loans."

That suggest to me that you've set up this loan as a downloading loan, (i.e., entries to the loan Account are coming via downloads from the lender), as opposed to a manual loan where entries to the loan Account come from the principal payments you enter into your checking Account.  With downloading loans there is no loan register available to you.  

As @NotACPA has said, Quicken doesn't really handle auto loans particularly well but I wouldn't expect the difference between your calculation and the Account's loan balance to be that far apart.  If you go to the lender's site to check on the loan balance does it agree to the loan balance in the Account or your calculation?

You might want to deactivate that loan and convert it to a manual loan so you can get to the loan register.