Is it ethical to use an abandoned bicycle in my parking garage? I've recently moved to a new city and need a bicycle.
I noticed an unlocked bike covered in dirt with flat tires in my apartment's parking garage.  Would it be ethical to clean it up and use it for my commute?  I would leave it back  in the same place at the end of each day.
I don't get your logic.  So you would share with the rightful owner?

Borrowing is stealing.

When is property abandoned is a legal question.

I am not an attorney and don't take this a legal advice but to me a unlocked bike in public or semi public place for more than 30 days is abandoned.  

I seriously doubt you can just air it up and ride it.  On an economical level if you have to put new tires on it you are not getting a good return.  The rightful owner or a person with even less ethics than you will take it.  

Look for a used cheap bike. But then you don't know if you are buying a stolen bik