“Because you have a low credit score, your interest rate would be high if you applied on your own. Therefore, it makes sense to have your fiancée cosign if she feels comfortable doing so.
Keep in mind that your credit score isn’t the only determining factor in getting approved. Income, debt-to-income, and loan-to-value all play a huge role in the approval process. If you make enough income, you should be fine.
You should also have a frank discussion with your fiancée about what will happen to the car loan if you were to split before you’re married. It won’t be comfortable, but getting something in writing will save you a headache just in case the relationship goes awry.”