One thing to check is how you have the milage computer set up. Would recommend setting to reset on start /stop.
The next to keep in mind that a true CR or EPA city estimate is a defined mix of rural driving with a certain amount of stops over precise milage, not stuck in bumper to bumper traffic or stop lights at every block. Highway has its own definitions.
I only really pay attention to highway driving averages and there is a milage guage in addition to average number is displayed. You will see for example that there are brief times where milage gauge will show as much as 50mpg,
As to averages on 30 minute to 5 hour drives i have taken which have not been diluted too by the local portion of the trips I see 24 to 29 MPG and the most common averages I see are 25, 26, 27 MPG, I believe these numbers are pretty close. My driving speed on most highways 60s and 70s generally. "True City driving is going to rot in any car"