What the dash readout shows at any one point in time is really not relevant, the only relevant number is what it will do over the course of a fillup.
I got 35.7 MPG for a measured fillup running on a totally interstate trip with the cruise set at 67 mph for most of the trip.
I'm sure I could have done quite a bit better at 55, probably 2-3 mpg better at least.
Ford proved the Mustang can get even better than that, they got like 43 mpg or something like that when they set out to do a max fuel economy run at Bristol Motor Speedway, they ran an average of like 47 mph though to do it.
That was with an automatic car, my mileage was with a manual.
For the time I had my V6 manual, my overall MPG average was 29 mpg for all the miles on the car.