To get back to the OP's question, I don't think it's a matter of "percentage of your income" or even "percentage of your disposable income".
It's a matter of how much of your disposable income are you comfortable tying up for a set amount of time. You also have to account for the additional repair/maintenance costs to be incurred. For me, I am finally in a place where I HAVE disposable income. I like being able to go out to lunch, etc. without checking my bank balance. So for me, a smaller payment than average is preferred, so I don't change my current situation. For others, that may be different.
