I only gotten over 450+ in 1 thank once, that was when the car was completely 100% stock, I did a road trip to las vegas at a steady 70mph with no traffic all the way both ways.
trick was I fueled up, reset my trip A on the dash, left home at midnight and got there around 5am, did the same on the way home a few days later, I calculated the mpg at 38+ at the gas pump. it all depends what kind of driving you do and how you drive. I drive mostly highway so I usually average 29-31mpg driving to work.