My grandfather once told me I should be spending 1 years income on a car and 5 years income on a home, and that these were the 'right numbers'.
He was a wonderful man, but full of **** financially.
No particular rule, but single digits. I am also an itinerant worker, and so get considerable tax benefits from driving a newer vehicle.
I made a taxable gain on my last car.
I don't tend to look at car buying in terms of % of net worth, but rather in terms of cashflow.
These days I manage pretty much everything in terms of cashflow, figuring that if my strategy and cashflow are correct, everything else will follow.