If it's not your first car on your report, you might get single digits (albeit higher ones) from the dealership, but if you fall into the first time category as a result of your report, you're probably looking double-digits for the auto loan.
That all said, if you're making progress on your report, six months is roughly enoguh to negate the first time buyer penalty when it comes to auto-enhanced credit scores, so you can refinance pretty easily at that point. With 20K down you're not going to have a problem getting the loan, it's just a matter of short-term interest. I think the most defensible position since you're in kinda an inbetween place, would be to line up financing with a lender first before walking into the dealership and let them try to beat it. Based on personal experience and that of numerous members of this forum, if you're a member of a CU, go there first absolutely... not all are created equally, but many are flat out better than dealer financing especially when it comes to used vehicle financing.
Starting Score: EQ 5 561, TU 98 567, EX 2 599 (12/30/11) Current Score: EQ 5 771, TU 4 741, EX 2 776, EQ 8 809, TU 8 774, EX 8 822 (1/1/18) Goal Score: EQ 5 771, TU 4 750, EX 2 776, EQ 8 809, TU 8 774, EX 8 822 (01/01/19)