I have to agree with hellokit.
I am also somewhat obsessive with things like car maintenance, finances, and a few other things.
I have attempted to help friends, relatives, and significant others with car maintenance and with finance-related issues simply because I do know more about these topics than they do.
When they do something (or fail to do something) that will be self-defeating in the long run, I want to help them. I explain that taking a particular action will save them money in the long run, or that they can extend the life of their car, or that they can avert a mechanical breakdown if they do what I recommend. In most cases, they do not want to do what I have recommended. Then, I have usually ratcheted up the force of my recommendation, simply because I know that they are being ultimately foolish and short-sighted. And, in almost every case, this has led to a high level of tension.
Trust me, even though you are trying to do the right thing, it is not worth the effort. Someone who is prudent will take good advice, such as you are giving, but many people are not prudent.
People who don’t understand the issue–be it car-related, or finance-related–have to learn what happens when they take a certain action, and they have to learn what happens when they fail to take action. Sad to say, many of these people will only learn when they are finally hit with the consequences that we knew were coming.
Unless you are already financially tied to your fiance, then I suggest that you jut let the chips fall where they may with her car. To “butt heads” with her in an attempt to be proactive and prudent will only frustrate you and will endanger your relationship.
Chill out, let her do what she wants, and when the inevitable happens, try to calmly explain how that could have been avoided. Just be sure to avoid the “I told you so” mode.