You should do manual calculations. First, fill tank to when it clicks off ( never try to squeeze more in, that is a bad thing in the world of good versus bad) and set trip odometer to 0. Drive to a tank level you feel comfortable with, closer to empty is better. Then fill again and divide miles driven by number of gallons. Repeat a couple times and add all results together and divide by the number of occurrences. This will give you a truer average.
As far as your number, that could be accurate if you drive in city, with a lead foot, tow a lot, idle a lot, carry a lot of gear, always drive with defrost or A/C on, etc. The uniqueness of each person's driving habits makes it hard to compare numbers, but your numbers could be indicating a problem somewhere. Get your manually calculated numbers and let us know, might be able to talk through things here. Good luck.