<human>: Can you give me the gist of the text in a nutshell?

I use my boyfriend's car frequently to go to work, run errands, or go out. If I had to guess, I would say that I average out putting on about 60%-70% of the miles on his car. 

He has declined taking payments from me for his car and insurance (I initiated this conversation more than once in the past). His reasoning is he paid cash for his car and pays one lump sum for insurance at the beginning of the year. This is very generous of him and I do my best to be generous in other ways right back to him. In order to contribute I elected to pay for all gas(premium), most oil changes, and most car washes. 

The car is still very much so my boyfriend's car as I typically ask to use it and his need for it trumps mine.

I can tell something is up with his car. There is a delay in gear shifting and it's really "rough" in idle. There has also been an issue with it starting after putting gas in the tank for a few months. My boyfriend has taken it to his mechanic who said it was nothing to worry about/sort of shrugged it off. I would like to take it to my parents' mechanic for a second look as these concerns are overdue with being dealt with.

My question is this: How should we split paying to fix his car? I use it frequently enough to contribute at least half of maintenance bills but I'm worried my boyfriend putting off getting his car looked at/fixed will result in complications and a bigger bill. 

**To be clear: I want to help pay for maintaining his car, I am unsure of how to fairly contribute when he is shrugging general maintenance off until it turns into expensive repairs.**

Yes, we should have spoken about this before I started using his car but we didn't and this is the situation we're in now. We're both reasonable people and this isn't a point of tension. I will be speaking with him tonight about devising a plan moving forward but I would like some feedback from /r/relationships about this situation as I've never shared a vehicle in a relationship before.
<bot>: I use my boyfriend's car frequently. He has been a little slow in getting his car looked at/maintaining it properly and I am unsure of how to fairly share the cost incurred as a result.