As a Mechanic, an overinflated tire is less dangerous than an underinflated tire. Reason being, sidewall flex. If the sidewall flexes too much, too much heat will generate, and the *potential* for a blowout is increased.
I've been maintaining 38 psi (cold) on all my vehicles for the past 20 years. Depending on ambient temperatures, the target *operating pressure* is about 42 psi (hot). You get nice, even wear across the face, and, yes, you're maximizing mpg.
The tires come overinflated from the factory for two reasons... 1) to prevent flat spots in the tires, and 2) to ensure the bead is properly seated.
You can safely maintain 38 psi with no issues... you'll never exceed the load rating with the Niro.
Fuel economy is mad sexy...