I have a petrol auto myself, and have another two petrol V6 manuals at work.
My auto gets around 500km out of the 80L tank, about 50/50 highway and city driving.
the manuals get a LOT better economy, around 500-600km for the same driving, however the clutch and gearbox for the manuals is quite clunky and indirect whereas the auto is silky smooth and very punchy.
They are very toey engines though, and very smooth (I believe they are basically the same engine as 350 and 370Z sports cars). Anything else you'd like to know?