How are you measuring your economy then?
Do this
Go fill your tank at a regular gas station you usually go to. Top the tank off and hit your trip reset button.
Drive around, when it gets to 1/4 tank, go back to SAME gas station and go to SAME pump. Fill your tank again and then divide the mileage by the gallons used.
Do this a few times, and remember to use the same pump each time. Reason being different gas pump nozzle shut off at different times. You may go to one that tops your tank off, but go to another one that shuts it off 3/4 gallon sooner.
There are other factors here, like driving different routes and having a head or tailwind on the highway than can sway your numbers, but we can ignore them.
I just find it tough to beleive you are getting 10MPG without something mechanically wrong.