Of course, if the recommended is 32 are at 20, that is huge drop. that is about 33% less air than recommended. And your 30 psi is low to but not too bad.
That is one reason why they made it mandatory for new cars starting in year xxxx ( I don't remember the exact year) that all new cars must have TPSM. This was to help to save fuel.
It was model year 2007 that all vehicles were required to have Direct Tire Pressure Monitoring Systems installed. The reason wasn't for fuel mileage, it was because the large majority of American owners are incompotent and irresponsible compared to the older days where taking care of your vehicle was expected. Remember the whole Ford Exploders with the Firestone deal? A large factor in that was tire pressure and the fact that today's generation of automobile owners don't even know how to change their spare tire let alone where the air goes in on a tire.
Its an "electronic nanny" that was implemented because the irresponsible, incompotent, lazy, leeching, sue-happy American public kept getting themselves hurt and/or killed because they couldn't be bothered to be responsible with heavy, potentionally dangerous machinery. I was taught when you EARN the right to drive and own a vehicle you take care of it; you check your fluids, air pressure, air filter, mirrors, condition of vehicle, operation of vehicle, ect. regularly. You don't need to be a mechanic, just responsible; something the past decade or two of vehicle owners lack and/or don't care because they can have "someone else do it" or sue the company when they endager themselves or someone else because of their incompotence.
I don't apologize for sounding rude/cranky, when you come into a dealership or shop and tell me "my tire light is on, I don't know why..." and your tires are flat as hell and the fancy screen in your dash is saying "HEY YOU GOT X TIRES WITH LOW PRESSURE!" you have automatically lost any respect and/or acknowledgement of any possible braincell that's rattling around in that cocunut on your shoulders from me. As a technican is sickens me that I or the dealership have no real power in preventing an idiot who's refuses to change their bald, cord showing tires from leaving the dealership and probably spinning out on the freeway and hurting someone else (because honestly at this point, Darwin's theory is just not fast enough in my honest opinion) and their families.
EDIT- ...Sorry I got a bit carried away there, its just today's lack of respect, self-responsibility, and common sense in people grates my nerves to no end... this is not aimed at anyone in particular and the people who should be offended (not that they deserve to be) are the ones who are in fact incompotent, irresponsible, lazy, and sue-happy.
In response to the OP, yes significantly low tire pressure can reduce over all mileage significantly. I would highly suggest inspecting that tire for leaks, and or damage that could be causing it to lose pressure; followed by regularly checking your tire pressure at leat every two or three weeks.
