AIBU to tell my boyfriend to stop using my car?

[Ozzielass]

I have just moved in with my long term boyfriend

he drives a van for his work and I have my own car

everytime he runs an errand after finishing work, he just assumes he can take my car

I understand that his van probably costs more to run, as it’s bigger … but my car is my pride and joy and he doesn’t ask - he just takes the keys and before I notice, I see my car has gone.

am I being unreasonable to ask him to stop using my car as his ‘run around’ car ?

not only this, but he moved the mirrors, the seats and wears his work boots which leaves my car dirty

This is a short-sighted attitude. You have become a team, so resources are pooled and shared to maximise what you have/reduce your outgoings. As a household, you are better off if he uses the car for short errands. Instead you are resource-guarding.
Think ahead a few years and you might find yourself on maternity leave and needing him to 'share' his income. Let's hope he doesn't have the same 'yours and mine' attitude then...

To clarify we are older and will not be having children together

perhaps I AM resource guarding - I have suffered through an abusive marriage and this is my first relationship since. My car was bought despite financial abuse and was the first thing ‘I’ have ever owned.

that is why it’s my pride and joy and BF knows this

I'm guessing Op that maybe you wouldn't be quite so upset about this if he first of all just said "OK if I take your car?" just as a courtesy, and then he didn't wear his dirty work clothes and boots to knowingly mess it up inside and at the end he did his best to return your mirrors and seat to how he found them.
My partner brought me my car for a birthday present but he still treats it as mine. He knows I really look after it so he wouldn't deliberately or carelessly make it dirty inside. He also makes the gesture of asking if he can take it, even though he knows I won't object because he is considerate with it.

Not exactly the same, is it? But if there was a reason that he wanted or needed to wear my underwear instead of his, then of course I would let him. If he decided to wear them without asking then I wouldn't be worried about asking him why he had done that.

As it happens, I once borrowed a pair of his underpants when we were away and for some reason I hadn't packed enough pairs and he had overpacked. They were very comfortable!