I'm surprised that I haven't seen anyone address the intangibles of owning a car. Especially with all of the enthusiasts on here and all the talk of "appliances" vs. "performance" or "driver's cars" or "classics" or "collectors", etc.
There are really only two ways to look at it.
1. Financial. It's not how much you make, but how much disposable income you have or will have to work with, before and after the purchase. All the talk about percentages and debt ratios and buy vs. lease really just boils down to how much money you'll have in your pocket at the end of the month, or the end of the purchase, whichever the case may be. Also, it depends on what you need to be able to DO with that residual amount, i.e. what other obligations or desires/hobbies/interest do you have? Also, what are the opportunity costs? What else could that money be doing for you if it weren't used to purchase/lease/finance a vehicle? At the end of the day, almost all of us need some sort of reliable transportation. However, most cars in most circumstances for most people are bad *investments*, strictly from a financial perspective.
2. Psychological. How much are you willing to pay to derive whatever psychological benefits you expect to realize? Whether it's driving pleasure, status, keeping up with the Joneses, speed, quality, technology, you're a car collector, vintage and classic autos, racing, car shows, or whatever other intangibles. Some are willing to pay a lot for any or all of those things, so it can't be simply boiled down to a dollars and cents argument to those people.
So to answer the OP question, I don't think it can be reduced to a fixed or maximum percentage for everyone. There are just two many variables. To me, the bottom line is that a car can be a bad *investment* without necessarily being a bad *purchase*.