My wife drives an '08 Hyundai Azera Limited 3.8L with ~79k miles. Over the past few months she has been complaining that the gas mileage has gone down. Normal around town driving was getting us ~19.6 mpg (via the in dash meter). Recently fuel mileage is down to an even 17 mpg.
I started doing some diagnostics on the car to see if I could figure out what's going on. Using my trusty Innova 3160d scan tool, I used the live data mode to check the O2 sensors to see how they were behaving. They looked quite normal with readings all over the board between .100 & .900 mv. I started looking through the other readings which were available.
The Hyundai provides about 38 different items you can check through the DLC port (and I can see on this scanner). The only thing which caught my eye was that the barometric pressure was showing at 29.2 in/hg. The reason it caught my eye was that it didn't move. I didn't think it would move a lot, but maybe it would move some. With scanner in hand I started testing the other vehicles in my stable, only to find out neither of them showed the same reading. I had to go to the next door neighbors house and "borrow" their cars. I finally found one which showed a barometric reading. The vehicle I checked showed a pressure of 29.8 in/hg.
While .6 in/hg does not appear to be a large amount, in the world of climatology I believe it is fairly significant. What I'm very curious about, though, is if the car is showing this much of a difference from actual, how would it affect fuel mileage? Would it be enough to drop it like I describe above? Yes, I'm assuming the neighbor's vehicle to be reading correctly as it is a much newer vehicle than mine. It could be off, but I'm highly doubting it at this point.