You can get a lot of information from your risk department. The credit bureaus have documentation to support a higher delinquency/default rate based on credit scores. You should be able to identify (and document) higher repair costs (Look at age and mileage both). Depending on the size of the bank, risk appetite and complexity, it could be something simple. Answer and document the question, WHY am I charging a higher rate?
If there is an application fee, consistent with all loans, it will affect the APR, but not the rate, Same with a shorter term loan, since the fee is spread over a shorter timeframe. Those are easy to defend.
Integrity. With it, nothing else matters. Without it, nothing else matters.