This is windhchaser's "blog", there's a ton of separate threads. Most of them in foo.
30 psi on a 5 inch tire is like 150 psi in a 25 mm road bike tire. That's extremely overinflated, enough to damage the tire and rim. Not to mention that it's terrible for comfort and handling.
See t
his article on "hoop stress", needed air pressure is proportional to the tire width. (like Shimagnolo posted above.)