The mileage you're seeing does seem a bit low, but it isn't wildly out of line with some of the other drivers here. A couple of possible contributing factors, besides the mostly-highway commute, would be

* systematic error in the trip computer measurements (some of us see a +/- 2MPG error on this, others see almost none). Maybe yours is reading low.

* air conditioning (or, I suppose, heater) running full blast

* high speed highway driving (mileage drops off sharply as speed increases).

* ethanol in the local gas supply could hurt mileage

If you want to get a feel for what kind of mileage the car is capable of, take it for a 20-30 mile drive on some suburban roads (somewhere you can safely drive 35-45mph, without too much traffic). Especially if the engine is already warm, I think you'll find the mileage average increasing.

Did you reset the trip computer's average mileage when you refilled? If not, at least some of that 31mpg is carry-over from your first tank.