YANBU
The reason you are not being unreasonable is because he is inconveniencing you, by taking the only vehicle that you are able to drive, without checking with you first. If he actually asked, would you mind so much?
We have a car each - I 'look after' both because he is shit at remembering to get MOTs, services, insurance, done etc, but he officially owns both, however mine is mine and his is his.
We have a joint account and all car stuff comes out of that. My car is always topped up, because that's how I roll. His is always generally close to empty. Mine is bigger and more reliable, so he will ask use it if he needs to do a longer drive. He will also ask to use mine if his is empty and he needs to pick up the kids. Which is often. Mine is easier for me to use with my dodgy hip (its not automatic, its just less hard going on the leg because of the seat position) so if he took it and I needed it, then it would be inconveniencing me. However he always checks, never just takes it, although officially it belongs to him. We're a family though, so its all family stuff.
Your partner seems to think that because yours is the runaround he can run it into the ground where his car is more 'precious'. Honestly I don't see an issue with that if its true, but its his lack of consideration that would get my goat. What if you need to pop into town? you can't. Because your cars aren't interchangeable. That would get my goat too.