I've never heard 20% before, seems unreasonably low given reality. The calculation I've heard cast about more commonly is based upon monthly cost and can be used as a comparable to alternatives, e.g. public transportation. The numbers I've typically seen range between 25% to 30% of your take home pay.
Regardless, new cars don't cost $34,000 unless you want them to. This is nothing but ad bait capitalizing on poor understanding of median vs. average. Nice sedans, e.g. Honda Civic, or Toyota Corolla, start around $17-18K and there are econo-box offerings e.g., Nissan Versa, Ford Fiesta, etc., that start around $13-14K. You'd have to be in a pretty bad way to not be able to budget one of these either on payments or lease. Even then you can get some pretty nice used cars for half that if that's where you're at.
"Your mother was a hamster, and your father smelt of elderberrys!" -- Monty Python and the Holy Grail