Yup, she'll need to co-sign, but other factors can still apply.
For example, I had 41% DTI and excellent credit when I bought my new car. I am on the mortage of a house I no longer own (divorce). I had no revolving debt so it was literally all of my debt. The bank did not hesitate to give me $20k at 3.41%.
It just depends on the full picture.
Remember, an auto loan is not necessarily like a mortgage loan where they always use the lower mid-score. Some lenders will take the highest score of the two applicants. Not sure why you are considering not having your fiance on the loan, but if it is because of credit issues, it may not matter, and it may be a good way for her to get a positive tradeline.
You may be able to just state how much YOU pay in mortgage similar to a situation of renting with a roommate. The bank may want to see your bank statements showing your only paying half or a deposit every month of her half going into your account or if it is joint, the extra income coming in from your fiancé. If she doesn't have good credit, it could drastically affect your interest rate. So I would tell them your situation to see if there is a work around...no guarantees as they usually want the other half on the loan but they may just approve you by yourself with the lower DTI AFTER they run the app jointly and can see her financials along with yours. If she has great credit, I would just do a joint app.
It could, but not with all lenders. Some lenders use the highest score of the two applicants. OP can ask the lender which way they do it. And in any event, you can have them run the numbers with both people on the loan and with only one person on the loan, and see whether it comes back approved and with the same interest rate.