% of Income/Car Costs – Guidelines?

Although there are well known guidelines for the appropriate percentage of income to spend on housing cost (one would have though banks would have consulted them before making bad loans – but I digress), I have not found any that apply to car payments.

Because my wife and I have paid off one of the cars, our current (car payment/take home income) percentage is about 4%. If I figure in insurance, taxes and maintenance (but not depreciation), it comes in at 6.7%. To calculate take home pay for this exercise, I added my wife's paychecks to mine.