Computer or calculated
Mileage is a combination of many variables, the most important ones are speed and weather.
The thing is you must average many tanks of fuel over many thousands of miles to get an accurate calculation. This is because of the fuel foaming, and trapped air in the tank, and how much time you take to truely 'top it off'. Leave a gallon of air or foam in the tank and calculate your mileage!! It will seem pretty good, but it's not real.
And DON'T believe the overhead trip computer. I've had six of these CTD's and none of the overhead computers told the truth! Every one was optimistic by at least 1. 5 MPG when compared to the hand calculated miles/gallons.
I have had tanks that with careful 'topping off' at the first and second fill-up and a significant tailwind, and conservative driving [lots of police around, or the elderly folks in the truck] that have calculated out to be 20+ mpg, but this is a 'rigged' measurement. It's not real world
My trucks have all averaged 16 mpg around town and 17-18 on the highway. The way I drive now, and with the fuel I can put through the engine, I'm getting about 15. 5 most of the time. This is about 70% highway miles [probably averageing 85 mph, with many blasts to 100+]

I can't help myself!!
If you want great mileage, drive at about 60 mph, and don't try to maintain speed up hills, just let it kind of slowly decelerate, and then let the speed build up downhill. If you don't get killed in a 'Road Rage' incident, you will get great mileage.
For me I'm amazed that I can drive my truck [7400#] like I do and get several mpg better than my buddy with a 5. 2 litre dodge 1500 [5500# or so]. He just groans when we compare mileage. Much less the performance difference!
Greg L